Apply access flags for managed and unmanaged pages properly on ARMv6/v7

Apply access flags for managed and unmanaged pages properly on ARMv6/v7

When entering a mapping via pmap_enter() unmanaged pages ought to be
naturally excluded from the "modified" and "referenced" emulation.
RW permission should be granted implicitly when requested,
otherwise unmanaged page will not recover from the permission fault
since there will be no PV entry to indicate that the page can be written.

In addition, only managed pages that participate in "modified"
emulation need to be marked as "dirty" and "writeable" when entered
with RW permissions. Likewise with "referenced" flag for managed pages.
Unmanaged ones however should not be marked as such.

Reviewed by:	cognet, gber