Fix mountd's SIGHUP handler to set a variable of the correct type

[rmacklem]

Summary

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 "volatile sig_atomic_t", which appears
to be all that is needed to correct it.

Thanks got to Peter Eriksson for reporting this.

Test Plan

Mountd builds and handles SIGHUP correctly,
with the same behaviour as before the change.

[rmacklem]

Use _Atomic(int) instead of volatile sig_atomic_t
as recommended by David C. (theraven@).

[rmacklem]

Updated the diff as recommended by theraven@.