some more skeletal changes for virtual memory

added basic structures for swap, spage table, frame table, and memory mapping syscalls
memory mapping is implemented by the spage table, since that is already keeping track of sources, and the page fault handler will be checking all that anyway.  may as well store it all in the same structure

spage table does NOT currently check if a page is already pointed somewhere else, it just overwrites it.

page fault handler not yet written (still default code).  none of the functionality is actually used, just initialized in init.c right now.  making sure I don't break anything ....