mail: add volatile in grabh()

setjmp() requires that any stack variables modified between the setjmp
call and the longjmp() must be volatile. This means that 'saveint' in
grabh() must be volatile, since it's modified after the setjmp().
Otherwise, the signal handler is not properly restored, resulting in a
crash (SIGBUS) if ^C is typed twice while composing.

PR: 276119
Reported by: Christopher Davidson <christopher.davidson@gmail.com>
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/993

(cherry picked from commit 6c951b37170f1fb2ae8b4827070743e61b6eaed2)