Minor cleanups of str_collate fix.

Minor cleanups of str_collate fix.

* fns.c (str_collate): Move decl from here ...
* lisp.h (str_collate): ... to here.
* sysdep.c (str_collate): Prune away some of the forest of ifdefs.
Remove unnecessary casts.  Use SAFE_NALLOCA to avoid
potential problems with integer overflow.  Don't assume
setlocale succeeds.  Remove unnecessary test before restoring
locale via setlocale, and free the copied setlocale string
when done with it.

Fixes: debbugs:18051