Add vectors support to Fsort.

Add vectors support to Fsort.
* configure.ac (AC_CHECK_FUNCS): Check for qsort_r.
* src/fns.c (sort_vector, sort_vector_compare): New functions.
(sort_list): Likewise, refactored out of ...
(Fsort): ... adjusted user.  Mention vectors in docstring.
(sort_vector_predicate) [!HAVE_QSORT_R]: New variable.
* src/alloc.c (make_save_int_obj): New function.
* src/lisp.h (enum Lisp_Save_Type): New member SAVE_TYPE_INT_OBJ.
(make_save_int_obj): Add prototype.
* test/automated/fns-tests.el (fns-tests-sort): New test.