The remote stub can implement monitor commands which are not

The remote stub can implement monitor commands which are not
known by gdb. Such monitor commands can take a long time
to execute. An example of this is the "leak_search" monitor
command implemented in the Valgrind gdbserver.

Currently, gdb will timeout on such a monitor command.
The remote stub however will continue to execute the
command and send the output later. Gdb and the remote
stub can then be desynchronised : gdb sends a packet,
and the reply read from the stub is a previous packet.

The change committed uses getpkt_sane to detect a timeout.
In this case, it continues the loop.
A QUIT; is inserted in the loop to allow the user
to stop handling the current command. possibly
still creating a desynchronisation between gdb and the stub
but that will be upon user request.