leave(1): Replace magic exit codes with standard macros

Signed-off-by: Faraz Vahedi <kfv@kfv.io>
Reviewed by: oshogbo
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1499

(cherry picked from commit f7a6cf3a020600b77795729d1c01780dae8e6d2e)