libiberty: Expose choose_tmpdir, and fix constness of return type
include/ChangeLog:
* libiberty.h (choose_tmpdir): New prototype.
libiberty/ChangeLog:
* choose-temp.c (choose_tmpdir): Remove now-redundant local
copy of prototype.
* functions.texi: Regenerate.
* make-temp-file.c (choose_tmpdir): Convert return type from
char * to const char * - given that this returns a pointer to
a memoized allocation, the caller must not touch it.