grep: fix case-fold mismatches between DFA and regex

The DFA code and the regex code didn't use the same semantics for
case-folding.  The regex code says that the data char d matches
the pattern char p if uc (d) == uc (p).  POSIX is unclear in this
area; the simplest fix for now is to change the DFA code to agree
with the regex code.  See <http://bugs.gnu.org/16919>.
* src/dfa.c (static_assert): New macro, if not already defined.
(setbit_case_fold_c): Assume MB_CUR_MAX is 1 and that case_fold
is nonzero; all callers changed.
(setbit_case_fold_c, parse_bracket_exp, lex, atom):
Case-fold like the regex code does.
(lonesome_lower): New constant.
(case_folded_counterparts): New function.
(parse_bracket_exp): Prefer plain setbit when case-folding is
not needed.
* src/dfa.h (CASE_FOLDED_BUFSIZE): New constant.
(case_folded_counterparts): New function decl.
* src/main.c (trivial_case_ignore): Case-fold like the regex code does.
(main): Try to improve comment re trivial_case_ignore.
* tests/case-fold-titlecase: Add lots more test cases.

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