lib/pager: Call _exit in signal handler, not raise

lib/pager: Call _exit in signal handler, not raise

The signals are registered without SA_RESETHAND, which means that the
same signal handler is called over and over again.

This just works because eventually, the waitpid call will fail, leading
to a suppressed error message (because stderr is already closed) and
then an _exit(EXIT_FAILURE) call.

Just call _exit(EXIT_FAILURE) directly to avoid unneeded and failing
system calls.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>