Make modules work again part 2 (final): Link the module build back into the

Make modules work again part 2 (final): Link the module build back into the
kernel build.  It should now be possible to build and install the modules
using the 'mobj', 'modules' and 'minstall' targets in /usr/src/sys/Makefile,
though the preferred way is to simply build a kernel using the config or
the buildkernel and installkernel mechanisms.

This commit also fixes a few minor #include path problems that came up during
testing.

Tested with: config/make/install, direct make/install, and with
buildkernel/installkernel.

The following modules are not yet being built by the new system (and were
built with the old system):  vesa.ko, fpu.ko, gnufpu.ko, ibcs2_coff.ko,
usb.ko, ng_mppc.ko, libiconv.ko.