gdbserver: Move detach code to its own function

gdbserver: Move detach code to its own function

The code required to handle the 'D' packet is non trivial, so move it
out to its own function.

The moved out code is identical, except for the call to strtol and some
breaks that became returns.

Tested manually, and by running gdb.base/*detach*.exp with
native-gdbserver and native-extended-gdbserver.

gdb/gdbserver/ChangeLog:

	* server.c (handle_detach): New function.
	(process_serial_event): Move code out, call handle_detach.