Make Idx a signed type, rather than possibly unsigned.
The unsignedness was not really buying us anything, since the code
overflows for other reasons before getting to PTRDIFF_MAX. Making
it signed allows us to use -1 and -2 with abandon, like libc does,
thus lessening the number of differences between gnulib and libc.
Also, it should help avoid gratuitous warnings like the one
reported by Nelson H. F. Beebe in: http://bugs.gnu.org/22702
* lib/regex.h (__re_idx_t): Remove. All uses changed to regoff_t.
* lib/regex_internal.h (SSIZE_MAX): Define if <limits.h> doesn't.
(IDX_MAX) [_REGEX_LARGE_OFFSETS]: Now SSIZE_MAX.
(REG_MISSING, REG_ERROR, REG_VALID_INDEX, REG_VALID_NONZERO_INDEX):
Remove. Revert all uses to their libc versions.