fix to_open debug setting

fix to_open debug setting

This is a follow-on to the patch to auto-generate target debug methods.

While working on that patch I noticed that the to_open debug setting
will never work.  There is no path by which debug_to_open can be
called.

This patch fixes the problem by using a generic function as the
implementation of the various "target" subcommands, and then putting
the debug printing there.

This is also a tiny step toward fixing PR 7250 (and apparently why
command contexts were introduced).

Built and regtested on x86-64 Fedora 20.

2014-07-30  Tom Tromey  <tromey@redhat.com>

	* target.c (open_target): New function.
	(add_target_with_completer, add_deprecated_target_alias): Use
	set_cmd_sfunc, set_cmd_context.
	(debug_to_open): Remove.
	(setup_target_debug): Update.