regex: copy back from Gnulib
Copy regex-related files back from Gnulib, to fix a problem with
static checking of regex calls noted by Martin Sebor. This merges the
following changes:
* New macro __attribute_nonnull__ in misc/sys/cdefs.h, for use later
when copying other files back from Gnulib.
* Use __GNULIB_CDEFS instead of __GLIBC__ when deciding
whether to include bits/wordsize.h etc.
* Avoid duplicate entries in epsilon closure table.
* New regex.h macro _REGEX_NELTS to let regexec say that its pmatch
arg should contain nmatch elts. Use that for regexec, instead of
__attr_access (which is incorrect).
* New regex.h macro _Attr_access_ which is like __attr_access except
portable to non-glibc platforms.
* Add some DEBUG_ASSERTs to pacify gcc -fanalyzer and to catch
recently-fixed performance bugs if they recur.
* Add Gnulib-specific stuff to port the dynarray- and lock-using parts
of regex code to non-glibc platforms.
* Fix glibc bug 11053.
* Avoid some undefined behavior when popping an empty fail stack.