rtos: Rewrite rtos_qsymbol() and fix auto-detect false positive

Matthias Blaicher submitted a patch at http://openocd.zylin.com/#/c/891/
to fix the false positive; when no RTOS was detected OpenOCD used the
last RTOS in the list.

While reviewing the code affected by Matthias' patch a rewrite seemed
appropriate, to make the code readable.

Matthias has abandoned his change and this change also fixes the false
positive.

Change-Id: Ic3327ccd036da52ba0a7e21ef93018205e74149c
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://openocd.zylin.com/895
Reviewed-by: Matthias Blaicher <matthias@blaicher.com>
Tested-by: jenkins

src/rtos/linux.c

@@ -1398,8 +1398,6 @@ static int linux_thread_packet(struct connection *connection, char *packet,
 
 			if ((strstr(packet, "qSymbol"))) {
 				if (rtos_qsymbol(connection, packet, packet_size) == 1) {
-					gdb_put_packet(connection, "OK", 2);
-
 					linux_compute_virt2phys(target,
 							target->rtos->
 							symbols[INIT_TASK].