seatd: Remove runtime socket path configuration

Configurable socket paths exist mainly to facilitate multiple parallel
seatd instances. However, the only valid use-case for running multiple
instances of seatd is testing during development, which can just as well
be done by changing SEATD_DEFAULTPATH at compile-time for test builds.

Remove the command-line argument in seatd for runtime configuration of
socket path, hardcode the socket path in seatd-launch, and change seatd
unlink/chmod/chown code to not run when started by seatd-launch.

This means that seatd-launch will now fail to start seatd if another
seatd instance is already running. The unlink code still runs when seatd
is started normally to assist in system crash recovery, but this may be
removed later if we deem it unnecessary.

man/seatd-launch.1.scd

@@ -17,9 +17,6 @@ seatd-launch - Start a process with its own seatd instance
 *-h*
 	Show help message and quit.
 
-*-s <path>*
-	Where to create the seatd socket. Defaults to a unique file path.
-
 *-v*
 	Show the version number and quit.
 

man/seatd.1.scd

@@ -24,9 +24,6 @@ seatd - A seat management daemon
 *-g <group>*
 	Group to own the seatd socket.
 
-*-s <path>*
-	Where to create the seatd socket. Defaults to `/run/seatd.sock`.
-
 *-l <loglevel>*
 	Log-level to use. Must be one of debug, info, error or silent. Defaults
 	to error.
@@ -42,6 +39,8 @@ such as displays and input devices in a multi-session, multi-seat environment.
 seatd operates over a UNIX domain socket, with *libseat* providing the
 client-side of the protocol.
 
+The location of the socket for seatd is set at compile-time.
+
 # SEE ALSO
 
 The libseat library, *<libseat.h>*, *seatd-launch*(1)