sed: use grep's DFA matcher to speed up regular expression matching

sed: use grep's DFA matcher to speed up regular expression matching

When possible, sed now uses grep's DFA matcher to match regular
expression, often resulting in a 10x speed-up.
* NEWS (Improvements): Mention it.  Move this section to the top.
* sed/local.mk (sed_sed_SOURCES): Add dfa.c, and sort file names.
(noinst_HEADERS): Add dfa.h, and sort file names.
* sed/regexp.c (dfaerror, dfawarn): New functions.
(compile_regex_1): Compile pattern with dfa.
(match_regex): Use dfa.
* sed/sed.h: Include dfa.h.
(struct regex): New member dfa.
* sed/dfa.c: New file, copied from grep.
* sed/dfa.h: New file, copied from grep.
* po/POTFILES.in: Add sed/dfa.c.