main: call gjs_dumpstack on aborts and traps and optionally on segfaults

main: call gjs_dumpstack on aborts and traps and optionally on segfaults

In order to debug issues triggered by JS code we might need to
see the stack of it, this is not normally visible in static stack traces,
thus we need to call gjs_dumpstack () before dying the process.

Intercepting signals SIGABRT, SIGTRAP (needed for catching fatal glib
errors) SIGFPE and SIGIOT by default, while introducing a new
'backtrace-segfaults' flag for the SHELL_DEBUG environment variable to
do the same on SIGSEGV and SIGBUS (this is a precaution to avoid that we
corrupt the stack for automatic errors trackers).

In any case after dumping the stack we raise the signal again, in order
to make the system aware of it.

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