Use SIGTRAP (via G_BREAKPOINT()) if G_DEBUG=fatal-warnings

Use SIGTRAP (via G_BREAKPOINT()) if G_DEBUG=fatal-warnings

On Linux with gdb, it's much more convenient to debug programs using
G_DEBUG=fatal-warnings if we send SIGTRAP instead of abort() by
default.  The default handler for both is to terminate the process.

In particular this makes it more easily possible to debug a warning
that's not the first in a program; you can skip past it and
go to the warning you care about.

The "aborting..." message is removed since it's no longer accurate,
and anyways was never very useful; crashes should show up in ABRT/apport
type crash catching systems.

https://bugzilla.gnome.org/show_bug.cgi?id=648423