change gdb to use BFD's "dwz" functions

change gdb to use BFD's "dwz" functions

BFD recently got a few functions related to "dwz" files.  This patch
changes gdb to use them, just to share a bit more code.

This changes dwarf2_get_dwz_file to possibly return NULL.  This
simplified a bit of code elsewhere.

Built and regtested on x86-64 Fedora 18.  I specifically regtested it
using my pending dwz test case.

	* dwarf2read.c (dwarf2_get_dwz_file): Return NULL if
	.gnu_debugaltlink not found.  Use bfd_get_alt_debug_link_info.
	(dwarf2_read_index, create_all_comp_units): Update.