linux: add fanotify(7) API bindings.

The `fanotify` API[0] is a linux-specific API for notification and interception
of filesystem events. In some ways it is similar to `inotify`, but with
different advantages/tradeoffs. It is particularly well suited to full
filesystem/mount monitoring (vs per directory) and for allowing/denying access
to files (`inotify` lacks this capability).

The `fanotify` API has been updated several times since it was enabled in Linux
2.6.37. Presently I've only included support for the original `fanotify`
features, and the `FAN_MARK_FILESYSTEM` addition made in Linux 4.20. There are
subsequent updates in 5.0 and 5.1 not covered in this initial commit.

This commit adds the relevant constants and types from
`uapi/linux/fanotify.h`[1] and two new functions (`fanotify_init`[2] and
`fanotify_wrap`[3]) to `src/unix/linux_like/linux/mod.rs`. While I believe this
API is also present on Android I have presently limited my attention to Linux.

Although this commit focuses on Linux 4.20.x's `fanotify` API/constants I have
skipped adding constants for `FAN_ALL_CLASS_BITS`, `FAN_ALL_INIT_FLAGS`,
`FAN_ALL_MARK_FLAGS`, `FAN_ALL_EVENTS`, `FAN_ALL_PERM_EVENTS` and
`FAN_ALL_OUTGOING_EVENTS` even though they are present in this kernel version's
headers. These defines were deprecated[4] in later releases with instructions to
not use them in new programs or extend them with new values. It would be a shame
for new Rust programs to use deprecated #defines!

[0]: http://man7.org/linux/man-pages/man7/fanotify.7.html
[1]: https://github.com/torvalds/linux/blob/d54f4fba889b205e9cd8239182ca5d27d0ac3bc2/include/uapi/linux/fanotify.h
[2]: http://man7.org/linux/man-pages/man2/fanotify_init.2.html
[3]: http://man7.org/linux/man-pages/man2/fanotify_mark.2.html
[4]: torvalds/linux@23c9dee#diff-4c9ca62be6bf38cc08f7ea9daf16e379

Author: cpu

src/unix/linux_like/linux/mod.rs

@@ -477,6 +477,21 @@ s! {
         pub len: u32
     }
 
+    pub struct fanotify_event_metadata {
+        pub len: u32,
+        pub vers: u8,
+        pub reserved: u8,
+        pub metadata_len: u16,
+        pub mask_align: u64,
+        pub fd: ::c_int,
+        pub pid: ::c_int,
+    }
+
+    pub struct fanotify_response {
+        pub fd: ::c_int,
+        pub response: u32,
+    }
+
     pub struct sockaddr_vm {
         pub svm_family: ::sa_family_t,
         pub svm_reserved1: ::c_ushort,
@@ -2417,6 +2432,56 @@ pub const IN_ALL_EVENTS: u32 = IN_ACCESS
 pub const IN_CLOEXEC: ::c_int = O_CLOEXEC;
 pub const IN_NONBLOCK: ::c_int = O_NONBLOCK;
 
+// uapi/linux/fanotify.h
+pub const FAN_ACCESS: ::c_ulonglong = 0x0000_0001;
+pub const FAN_MODIFY: ::c_ulonglong = 0x0000_0002;
+pub const FAN_CLOSE_WRITE: ::c_ulonglong = 0x0000_0008;
+pub const FAN_CLOSE_NOWRITE: ::c_ulonglong = 0x0000_0010;
+pub const FAN_OPEN: ::c_ulonglong = 0x0000_0020;
+
+pub const FAN_Q_OVERFLOW: ::c_ulonglong = 0x0000_4000;
+
+pub const FAN_OPEN_PERM: ::c_ulonglong = 0x0001_0000;
+pub const FAN_ACCESS_PERM: ::c_ulonglong = 0x0002_0000;
+
+pub const FAN_ONDIR: ::c_ulonglong = 0x4000_0000;
+
+pub const FAN_EVENT_ON_CHILD: ::c_ulonglong = 0x0800_0000;
+
+pub const FAN_CLOSE: ::c_ulonglong = FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE;
+
+pub const FAN_CLOEXEC: ::c_int = 0x0000_0001;
+pub const FAN_NONBLOCK: ::c_int = 0x0000_0002;
+
+pub const FAN_CLASS_NOTIF: ::c_int = 0x0000_0000;
+pub const FAN_CLASS_CONTENT: ::c_int = 0x0000_0004;
+pub const FAN_CLASS_PRE_CONTENT: ::c_int = 0x0000_0008;
+
+pub const FAN_UNLIMITED_QUEUE: ::c_int = 0x0000_0010;
+pub const FAN_UNLIMITED_MARKS: ::c_int = 0x0000_0020;
+
+pub const FAN_MARK_ADD: ::c_int = 0x0000_0001;
+pub const FAN_MARK_REMOVE: ::c_int = 0x0000_0002;
+pub const FAN_MARK_DONT_FOLLOW: ::c_int = 0x0000_0004;
+pub const FAN_MARK_ONLYDIR: ::c_int = 0x0000_0008;
+pub const FAN_MARK_INODE: ::c_int = 0x0000_0000;
+pub const FAN_MARK_MOUNT: ::c_int = 0x0000_0010;
+// NOTE: FAN_MARK_FILESYSTEM requires Linux Kernel >= 4.20.0
+pub const FAN_MARK_FILESYSTEM: ::c_int = 0x0000_0100;
+pub const FAN_MARK_IGNORED_MASK: ::c_int = 0x0000_0020;
+pub const FAN_MARK_IGNORED_SURV_MODIFY: ::c_int = 0x0000_0040;
+pub const FAN_MARK_FLUSH: ::c_int = 0x0000_0080;
+
+pub const FAN_MARK_TYPE_MASK: ::c_int =
+    FAN_MARK_INODE | FAN_MARK_MOUNT | FAN_MARK_FILESYSTEM;
+
+pub const FANOTIFY_METADATA_VERSION: u8 = 3;
+
+pub const FAN_ALLOW: u32 = 0x01;
+pub const FAN_DENY: u32 = 0x02;
+
+pub const FAN_NOFD: ::c_int = 1;
+
 pub const FUTEX_WAIT: ::c_int = 0;
 pub const FUTEX_WAKE: ::c_int = 1;
 pub const FUTEX_FD: ::c_int = 2;
@@ -3328,6 +3393,14 @@ extern "C" {
         path: *const ::c_char,
         mask: u32,
     ) -> ::c_int;
+    pub fn fanotify_init(flags: ::c_int, event_f_flags: ::c_int) -> ::c_int;
+    pub fn fanotify_mark(
+        fd: ::c_int,
+        flags: ::c_int,
+        mask: ::c_ulonglong,
+        dirfd: ::c_int,
+        path: *const ::c_char,
+    ) -> ::c_int;
 }
 
 cfg_if! {