remote: get rid of all the T packets when syncing the thread list

remote: get rid of all the T packets when syncing the thread list

This commit avoids the prune_threads call in the remote target's
target_update_thread_list's implementation, eliminating all the "thread
alive" RSP traffic (one packet per thread) whenever we fetch the
thread list.

IOW, this:

 Sending packet: $Tp2141.2150#82...Packet received: OK
 Sending packet: $Tp2141.214f#b7...Packet received: OK
 Sending packet: $Tp2141.2141#82...Packet received: OK
 ... more T packets; it's one per previously known live thread ...
 Sending packet: $qXfer:threads:read::0,fff#03...Packet received: l<threads>\n<thread id="p2141.2141" core="2"/>\n<thread id="p2141.214f" core="1"/>\n<thread id="p2141.2150" core="2"/>\n</threads>\n

Becomes:

 Sending packet: $qXfer:threads:read::0,fff#03...Packet received: l<threads>\n<thread id="p2141.2141" core="2"/>\n<thread id="p2141.214f" core="1"/>\n<thread id="p2141.2150" core="2"/>\n</threads>\n

Tested on x86_64 Fedora 20, native gdbserver:
  - tests the qXfer:threads:read method.

Tested on x86_64 Fedora 20, native gdbserver with qXfer:threads:read
force-disabled in gdbserver:
  - So that GDB falls back to the qfThreadInfo/qsThreadInfo method.

And also manually smoked tested force disabling both
qXfer:threads:read and qfThreadInfo/qsThreadInfo in gdbserver.

gdb/
2014-10-15  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (ALL_NON_EXITED_THREADS_SAFE): New macro.
	* remote.c (remote_update_thread_list): Skip calling prune_threads
	if any thread listing method is supported, and instead walk over
	the set of remote threads listed, deleting those that are not
	found in GDB's thread list.