main: use SA_NODEFER to track signals to being able to raise from alarm

main: use SA_NODEFER to track signals to being able to raise from alarm

After we receive one of the tracked signals, in case we get stuck inside
the gjs_dumpstack () call, we use an alarm to raise the previously emitted
signal, however without using SA_NODEFER, the raise inside the alarm handler
will be ignored.

To avoid to handle new signals caused by the handler calls, once we get the
first signal, we just ignore them all as they could only lead to dirty traces.

Also, cleaning up a bit the code, and disabling the shell log handler
in dump_gjs_stack_alarm_sigaction since this might lead to a new
gjs_dumpstack () request.

https://bugzilla.gnome.org/show_bug.cgi?id=789237