maint.mk: teach sc_prohibit_magic_number_exit to accept 77

maint.mk: teach sc_prohibit_magic_number_exit to accept 77

* top/maint.mk (sc_prohibit_magic_number_exit): Do not complain about
uses like "exit (77)".  "77" is automake's "skip this test" exit code.
It is not in the same category as "exit (0)" or "exit (1)", and
besides, I know of no symbolic name for that 77.  Reported by
Richard W.M. Jones in
http://thread.gmane.org/gmane.comp.emulators.guestfs/1902