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.

From-SVN: r216285