lib: Make pidfile_path_create() return the existing PID on conflict

lib: Make pidfile_path_create() return the existing PID on conflict

Use F_GETLK to get the lock holder PID, this is more accurate than
reading the file contents: A conflicting process might not have
written its PID yet. Also, F_GETLK easily allows to do a retry if the
lock holder just died.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>