Don't let SIGINT handling block SIGCHLD indefinitely.

Don't let SIGINT handling block SIGCHLD indefinitely.

* atimer.c (block_atimers):
* callproc.c (block_child_signal): Block SIGINT too;
otherwise, its handler might now unblock signals that it shouldn't.
* keyboard.c (read_char): Clear signal mask, since we may
be in a SIGINT handler, and many signals may be masked.
* keyboard.c (handle_interrupt):
* sysdep.c (handle_arith_signal):
Clear signal mask instead of just unblocking the signal that
was received, since several signals may be blocked at this point.

Fixes: debbugs:17561