configure: Add --with-modulesdir to accompany --with-libdir.

configure: Add --with-modulesdir to accompany --with-libdir.

This starts the seplitting of libdir in to libdir and modulesdir.
Our shared libs should go into libdir, the internal shared modules,
codepages, and other stuff that was originally in libdir, should
go into modulesdir.

The idea behind this is, that in a typical installation,
the shared (and static) libraries (as libtalloc, libsmbclient,
libwbclient and others) should be put into /usr/lib, while
the e.g. the vfs modules should reside in /usr/lib/samba.

This is meant to ease the work of packagers and reduce
the needs for manual interaction and workarounds.

Michael