[$] Sharing page tables with msharefs

A page-table entry (PTE) is relatively small, requiring just eight bytes to refer to a4096-byte page on most systems. It thus does not seem like a worrisomelevel of overhead, and little effort has been made over the kernel'shistory to reduce page-table memory consumption. Those eight bytes canhurt, though, if they are replicated across a sufficiently large set ofprocesses. The msharefspatch set from Khalid Aziz is a revised attempt to address thatproblem, but it is proving to be a hard sell in the memory-managementcommunity.