Improve robustness of new string-collation code.

* configure.ac (newlocale): Check for this, not for uselocale.
* src/sysdep.c (LC_COLLATE, LC_COLLATE_MASK, freelocale, locale_t)
(newlocale, wcscoll_l): Define substitutes for platforms that
lack them, so as to simplify the mainline code.
(str_collate): Simplify the code by assuming the above definitions.
Use wcscoll_l, not uselocale, as uselocale is too fragile.  For
example, the old version left the Emacs in the wrong locale if
wcscoll reported an error.  Use 'int', not ptrdiff_t, for the int
result.  Report an error if newlocale fails.

Fixes: debbugs:18051

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