Fix =thread-exited not showing up when detaching (PR 15564)

Fix =thread-exited not showing up when detaching (PR 15564)

I sent a patch in 2013 for this (incorrectly named =thread-created):

  https://cygwin.com/ml/gdb-patches/2013-06/msg00129.html

Tom Tromey was ok with the change, but suggested to add a test as well.
Then I forgot about this patch until today. So here it is again, with the
corresponding test.

The problem is that the =thread-exited event does not appear when detaching
from a local process. It does appear with remote though. It's not a really
big deal, but I'd like it to be consistent.

Tested with local and remote Linux on my Ubuntu 14.04.

gdb/ChangeLog:

	PR gdb/15564
	* inferior.c (detach_inferior): Call exit_inferior_1 with silent = 0.

gdb/testsuite/ChangeLog:

	PR gdb/15564
	* gdb.mi/mi-detach.exp: New file.