mountd: Fix type of signal variable

Without this patch got_sighup(), which is the SIGHUP handler,
would set a variable of type int.

This would appear to be incorrect, although it has worked reliably.
This patch changes the type to "_Atomic(int)", which appears
to be all that is needed to correct it.

Reported by: pen@lysator.lui.se
Reviewed by: theraven, karels (prev version), kevans (prev version)
Differential Revision: https://reviews.freebsd.org/D41265