gdbstub: implement remote debugging protocol escapes for command receive

- decode escape sequences
- decompress run-length encoding escape sequences
- report command parsing problems to output when debug output is enabled
- reject packet checksums that are not valid hex digits
- compute the checksum based on the packet stream, not based on the
  decoded packet

Tested with GDB and QtCreator integrated debugger on SMP QEMU instance.
Works for me.

Signed-off-by: Doug Gale <doug16k@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
