Make --no-site-lisp work for --enable-locallisppath elements

Make --no-site-lisp work for --enable-locallisppath elements
without "site-lisp" in the name.
Ref http://debbugs.gnu.org/10208#25, point iii).

* configure.in (standardlisppath): New output variable.
(lisppath): Use standardlisppath.
* Makefile.in (standardlisppath): New, set by configure.
(epaths-force): Use standardlisppath and locallisppath rather than lisppath.

* src/epaths.in (PATH_SITELOADSEARCH): New.
* src/lread.c (init_lread): Use PATH_SITELOADSEARCH.

* nt/paths.h (PATH_SITELOADSEARCH): New.