Target remote mode fork and exec event support

Target remote mode fork and exec event support

This patch implements support for fork and exec events with target remote
mode Linux targets.  For such targets with Linux kernels 2.5.46 and later,
this enables follow-fork-mode, detach-on-fork and fork and exec
catchpoints.

The changes required to implement this included:

 * Don't exit from gdbserver if there are still active inferiors.

 * Allow changing the active process in remote mode.

 * Enable fork and exec events in remote mode.

 * Print "Ending remote debugging" only when disconnecting.

 * Combine remote_kill and extended_remote_kill into a single function
   that can handle the multiple inferior case for target remote.  Also,
   the same thing for remote_mourn and extended_remote_mourn.

 * Enable process-style ptids in target remote.

 * Remove restriction on multiprocess mode in target remote.

gdb/gdbserver/ChangeLog:

	* server.c (process_serial_event): Don't exit from gdbserver
	in remote mode if there are still active inferiors.

gdb/ChangeLog:

	* inferior.c (number_of_live_inferiors): New function.
	(have_live_inferiors): Use number_of_live_inferiors in place
	of duplicate code.
	* inferior.h (number_of_live_inferiors): Declare new function.
	* remote.c (set_general_process): Remove restriction on target
	remote mode.
	(remote_query_supported): Likewise.
	(remote_detach_1): Exit in target remote mode only when there
	is just one live inferior left.
	(remote_disconnect): Unpush the target directly instead of
	calling remote_mourn.
	(remote_kill): Rewrite function to handle both target remote
	and extended-remote.  Call remote_kill_k.
	(remote_kill_k): New function.
	(extended_remote_kill): Delete function.
	(remote_mourn, extended_remote_mourn): Combine functions into
	one, remote_mourn, and enable extended functionality for target
	remote.
	(remote_pid_to_str): Enable "process" style ptid string for
	target remote.
	(remote_supports_multi_process): Remove restriction on target
	remote mode.