GDB-Protocol: Formalize ``!'' extended remote

[Andrew Cagney]

Hello,

At present there are two general forms of the remote protocol ``remote''
and ``extended-remote'' (there is also extended - extended - unofficial
- confused - remote but that is another issue ;-).

At present the user must issue ``extended-remote'' before GDB will try
to use that command.  I'd like to propose that, instead GDB always try
for extended-remote (sending the ``!'' packet and if nack'd revert to
the old protocol.

An override mechanism would also be needed.

	Andrew