roken: add mergesort_r()

Add mergesort_r() as a stable sort function that can be used by other
components of Heimdal. Note that there is no standardized prototype for this
function, however it appears that both FreeBSD and glibc would adopt the glibc
convention (where the private data argument appears last). See:

    https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214248

lib/roken/version-script.map

@@ -87,6 +87,8 @@ HEIMDAL_ROKEN_2.0 {
 		rk_localtime_r;
 		rk_memmem;
 		rk_memset_s;
+		rk_mergesort;
+		rk_mergesort_r;
 		rk_mkdir;
 		rk_mkdtemp;
 		rk_mkostemp;