Simplify building libpam and fix libpam.a not containing the modules since r284345.

The change in r284345 moved the creation of openpam_static_modules.o to
lib/libpam/static_modules but never managed to get them into libpam.a.

Move this logic to lib/libpam/static_libpam and have it create a static
library for libpam.a The main lib/libpam/libpam will only create a
shared library. No redundancy in compilation or installation exists
in this solution.

This avoids requiring a pass with -D_NO_LIBPAM_SO_YET.

Sponsored by: EMC / Isilon Storage Division