Install a self-contained NS build's libexec directly into the right place

Install a self-contained NS build's libexec directly into the right place
This is rather than installing it in one place then moving it.

* configure.in (archlibdir): Set it for self-contained ns builds.
(libexecdir): Don't expand it now (this is mainly cosmetic).

* Makefile.in (ns_appbindir, ns_appresdir):
Move them before things that may refer to them.
(install-arch-dep): No need to relocate self-contained ns libexec.

* lib-src/Makefile.in (ns_appbindir): New, set by configure.