grep: fix new heap write buffer overrun

* src/dfa.c (parse_bracket_exp): Fix off-by-one allocation error.
Exposed by running the tests with an ASAN-enabled binary (i.e.,
created using gcc's -fsanitize=address option).  Introduced by
commit v2.18-70-gd3d9612, "dfa: simplify range char allocation".
grep: avoid wasting memory for large patterns in dfamust

* src/dfa.c (struct must): New member 'prev'.  It points to the
previous must.
(allocmust): New function.
(freemust): New function.
(dfamust): Use it.
dfa: minor tuneup of dfamust memory savings patch

* src/dfa.c (allocmust): Use xmalloc, not xzalloc.
Initialize the must completely, so that the caller need not
invoke resetmust.  All callers changed.
(dfamust): Omit asserts that aren't needed on typical machines
where dereferencing NULL dumps core.  Don't leak memory if the
pattern contains a NUL byte.
dfa: fix incorrect comment that led to heap overrun

* dfa.c (maybe_realloc): Fix comment to match behavior.
grep: skip checking of multibyte character boundary, reaching at eolbyte

* src/dfa.c (dfaexec): Skip checking of multibyte character boundary,
reaching at eolbyte.
dfa: simplify and be more consistent about MB_CUR_MAX

* src/dfa.c (struct dfa): New member 'multibyte',
replacing 'mb_cur_max'.  All uses changed.  Use this new member
consistently, instead of sometimes referring to MB_CUR_MAX directly.

dfa: fix comment
* src/dfa.c (maybe_realloc): Fix comment to match behavior better.
dfa: minor simplification of dfaexec

* src/dfa.c (dfaexec): Streamline updating of returned values.
Don't bother to check d->multibyte before updating mbp.
Avoid duplicate p > end test.
misc: fix doc and test bugs re grep -z

Problem reported by Stephane Chazelas in: http://bugs.gnu.org/16871
* doc/grep.texi (Usage): Remove incorrect example with -P.
* tests/pcre: Improve test so that it actually tests whether \s
matches a newline.
dfa: fix recently-introduced memory leak

Problem reported by Aharon Robbins in: http://bugs.gnu.org/17341
* src/dfa.c (dfasuperset): free after dfafree.