I just changed libc/gen/[u]sleep.c to be less susceptible to SIGALRMs
coming in too late when other signal handlers are also active.

I also realized that the change makes another race possible:
a previous SIGALRM handler might now miss a signal because the sihnal mask
is reset earlier. I can't see a way out of this without adding one more
system call (to the eight which are already there as smartly pointed out
by th eman-page).

Thoughts?

-pk