Add push_target overload

Add push_target overload

This adds a push_target overload that takes a "target_ops_up &&".
This removes some calls to release a target_ops_up, and makes the
intent here clearer.

gdb/ChangeLog
2019-02-15  Tom Tromey  <tromey@adacore.com>

	* target.h (push_target): Declare new overload.
	* target.c (push_target): New overload, taking an rvalue reference.
	* remote.c (remote_target::open_1): Use push_target overload.
	* corelow.c (core_target_open): Use push_target overload.