clarify --signal flag help

[alexbozhenko]

Expand the help to make it obvious what is going on for each command, to remove the need to look up it in the docs or read the code

    -sl,--signal <command>[=<pid>]   Send a command to a running nats-server:
                                       ldm - lame duck mode: evicts clients gradually, then shuts down gracefully (SIGUSR2)
                                       quit - shuts down gracefully (SIGINT)
                                       term - shuts down gracefully (SIGTERM)
                                       stop - terminates immediately (SIGKILL)
                                       reload - reloads server configuration (SIGHUP)
                                       reopen - reopens log files (SIGUSR1)
                                     <pid> can be either a PID (e.g. 1) or the path to a PID file (e.g. /var/run/nats-server.pid)file (e.g. /var/run/nats-server.pid)

Signed-off-by: Alex Bozhenko [email protected]

[sciascid]

SIGINT and SIGTERM are handled sightly differently, see server/signal.go:

				case syscall.SIGINT:
					s.Shutdown()
					s.WaitForShutdown()
					os.Exit(0)
				case syscall.SIGTERM:
					// Shutdown unless graceful shutdown already in progress.
					s.mu.Lock()
					ldm := s.ldm
					s.mu.Unlock()

					if !ldm {
						s.Shutdown()
						s.WaitForShutdown()
						os.Exit(0)
					}

SIGTERM has the additional check for ldm.
Would it be worth highlighting this difference in the help message?