User-mode GDB stub improvements - handle signals

Handle signals in the user-mode GDB stub.  Report them to GDB, and
allow it to change or cancel them.  Also correct the protocol numbering;
it happens to match Linux numbering for SIGINT and SIGTRAP, but that's
just good fortune.

Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Acked-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6096 c046a42c-6fe2-441c-8c8c-71466251a162