Use sigsetjmp and siglongjmp instead of setjmp/longjmp in order to be

Use sigsetjmp and siglongjmp instead of setjmp/longjmp in order to be
consistent in how we deal with the signal mask.  POSIX doesn't specify
what to do with the signal mask in setjmp/longjmp which has resulted in
the signal mask getting saved on *BSD and not getting saved on Linux.
Making the behaviour explicit via sigsetjmp/siglongjmp gives us
consistency and saves expensive sigprocmask() syscalls on *BSD.