libc: fix signal handling in system()

libc: fix signal handling in system()

When built without NPTL support (or for a sparc target), the system()
function doesn't conform to its specification. Namely, it uses signal()
to install/save/restore signal handlers, which may break applications
using custom handlers installed with sigaction(). In addition, it resets
the SIGCHLD handler to SIG_DFL instead of blocking the signal, which may
result in "lost" signals if a custom handler was installed.

Fix system() by replacing uses of signal() with appropriate calls to
sigaction() and sigprocmask().

Signed-off-by: Richard Braun <rbraun@sceen.net>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>