Reimplment matcher using Depth-First-Search(backtracking).

	2013-07-23  Tim Shen  <timshen91@gmail.com>

	PR libstdc++/53622
	PR libstdc++/57173
	* include/bits/regex.h: regex_match() and regex_search().
	* include/bits/regex_cursor.h: Fix _M_set_pos().
	* include/bits/regex_grep_matcher.h: add _M_dfs_match().
	* include/bits/regex_grep_matcher.tcc: Implement it.
	* testsuite/28_regex/algorithms/regex_match/extended/string_group_01.cc:
	New.
	* testsuite/28_regex/algorithms/regex_match/extended/string_group_02.cc:
	New.
	* testsuite/28_regex/algorithms/regex_search/basic/string_01.cc:
	Remove xfail.