vendor: Add init.d directory to clean_files.txt

Also fix and lint the directory.
This is our code, and as such, should be linted.

Author: Noah Gorny

clean_files.txt

@@ -70,3 +70,6 @@ completion/available/vuejs.completion.bash
 aliases/available/dnf.aliases.bash
 aliases/available/vim.aliases.bash
 aliases/available/git.aliases.bash
+
+# vendor init files
+vendor/init.d

vendor/init.d/preexec.bash

@@ -1 +1,3 @@
-source ${BASH_IT}/vendor/github.com/rcaloras/bash-preexec/bash-preexec.sh
+# shellcheck shell=bash
+# shellcheck disable=1090
+source "${BASH_IT}"/vendor/github.com/rcaloras/bash-preexec/bash-preexec.sh