maint: fix some unlikely wordsplit overflows

maint: fix some unlikely wordsplit overflows

* gnulib.modules: Add reallocarray.
* lib/wordsplit.c: Include stdckdint.h.
(ISDELIM, expvar, isglob, scan_word):
Defend against strchr (s, 0) always succeeding.
(alloc_space, wsplit_assign_vars):
Fix some unlikely integer overflows, partly by using reallocarray.
(alloc_space): Avoid quadratic worst-case behavior.
(isglob): Return bool, not int.  Accept size_t, not int.
(to_num): Remove; no longer used.
(xtonum): Clarify the code the bit.  Rely on portable
conversion to unsigned char rather than problematic pointer cast.