Restore signal handler after uloop_run()

Restore signal handler after uloop_run()

uloop_run calls uloop_setup_signals() to set up signal handling before the while
loop, but does not remove the signal handling after the loop has ended. This can
cause problems for for example applications using the ubus file descriptor in
their own event loops, and perhaps with their own signal handling.

This patch stores the signal handle that was in place when the initial
uloop_run() call was made, and restores the handle when this call returns.
For recursive calls, the signal handler is not updated.

One use-case I experienced was an application that subscribed to several ubus
objects and used the ubus file descriptor in its own event loop. Even though
ubus_register_subscriber() (which calls uloop_run()) had returned, the signal
handler was not removed. This caused SIGINT not to be caught by the application.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>