convert corelow to to_xclose

This converts corelow to be a to_xclose target.  It moves all the
corelow-related globals into a new subclass of target_ops and arranges
for core_open to push an instance of this.

This is preparatory work for multi-target.  After this patch, corelow
doesn't rely on global state and multiple instances can readily be
instantiated.

2014-07-29  Tom Tromey  <tromey@redhat.com>

	* corelow.c (struct core_target_ops_with_data): New struct.
	(core_vec, core_gdbarch, core_data): Remove.  Now fields of
	core_target_ops_with_data.
	(sniff_core_bfd): Add core_gdbarch parameter.
	(get_core_target_ops): New function.
	(core_xclose): Rename from core_close.  Update for
	core_target_ops_with_data.  Free the target.
	(core_close_cleanup): Rename parameter.  Update.
	(core_open): Use TARGET_NEW.  Update.
	(get_core_register_section, get_core_registers, core_files_info)
	(core_xfer_partial, core_read_description, core_pid_to_str): Use
	get_core_target_ops.  Update.
	(init_core_ops): Set to_xclose, not to_close.
	* target.c (allocate_target): New function.
	* target.h (allocate_target): Declare.
	(TARGET_NEW): New macro.