tmpdir: port to VMS, to // != /, and to long dirs

tmpdir: port to VMS, to // != /, and to long dirs

* lib/tmpdir.c (__libc_secure_getenv) [!_LIBC]: Rename from
__secure_getenv, so that we're more like the glibc version.
All uses changed.
(path_search): Don't put slash after directory if __VMS.
Problem reported by Steven M. Schweda in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-07/msg00019.html>.
Do not remove trailing slash from "//".
Do not assume dlen <= INT_MAX.