Redefines PTE present vs mapped

Mapped now means it points to a physical page in all cases.  Present is
mapped and with bits set for some valid PTE walk.  For instance, a user
read works on x86 with PTE_U and PTE_P.

PTE_P shouldn't be used from the arch-indep code now.

I might have missed something with this commit, like leaking memory or
dirty bits.  Feel free to check out the usages of is_present, is_mapped,
and is_unmapped.