Change approach to missing manpage in net/intel-ix-kmod

Instead of not installing the missing man page - use the one from the previous
version.

Changes include:

• Revert all the MANPAGEEXISTS-related changes from net/intel-{ixl,ix}-kmod
• Allow slave ports of net/intel-ixl-kmod to set their own MASTER_SITES
• Add previous version of intel-ix-kmod to it's DISTFILES
• Add pre-install target to intel-ixl-kmod to place the man page where do-install expects to find it