Improve gdb.threads/attach-many-short-lived-threads.exp timeout handling

Improve gdb.threads/attach-many-short-lived-threads.exp timeout handling

The buildbot shows that this test is still racy, and occasionally
fails with time outs on some machines.  I'd like to get major issues
with load out of the way.

The test currently exits after 180s, which is just a random number,
that has no relation to what the .exp file considers a time out.  This
commit makes the program wait a bit longer than what the .exp file
considers a time out, and, resets the timer for each iteration.

Tested on x86_64 Fedora 20, native and extended-remote gdbserver.

gdb/testsuite/
2015-02-06  Pedro Alves  <palves@redhat.com>

	* gdb.threads/attach-many-short-lived-threads.c (SECONDS): New
	macro.
	(seconds_left, again): New globals.
	(main): Wait seconds_left in a 1-second sleep loop instead of
	sleeping 180 seconds.  If 'again' is set, reset the seconds
	counter.
	* gdb.threads/attach-many-short-lived-threads.exp (test): Set
	'again' in the inferior before detaching.  Print the seconds left.
	(options): New global.
	(top level): Build program with	-DTIMEOUT=$timeout.