make "set debug target" take effect immediately

make "set debug target" take effect immediately

Right now, "set debug target" acts a bit strangely.

Most target APIs only notice that it has changed when the target stack
is changed in some way.  This is because many methods implement the
setting using the special debug target.  However, a few spots do
change their behavior immediately -- any place explicitly checking
"targetdebug".

Some of this peculiar behavior is documented.  However, I think that
it just isn't very useful for it to work this way.  So, this patch
changes "set debug target" to take effect immediately in all cases.
This is done by simply calling update_current_target when the setting
is changed.

This required one small change in the test suite.  Here a test was
expecting the current behavior.

Built and regtested on x86-64 Fedora 20.

2014-08-04  Tom Tromey  <tromey@redhat.com>

	* target.c (set_targetdebug): New function.
	(initialize_targets): Pass set_targetdebug when creating "set
	debug target".

2014-08-04  Tom Tromey  <tromey@redhat.com>

	* gdb.texinfo (Debugging Output): Update for change to "set debug
	target".

2014-08-04  Tom Tromey  <tromey@redhat.com>

	* gdb.base/sss-bp-on-user-bp-2.exp: Expect output from "set debug
	target 0".