Avoid clearing EXEC permission bit when setting the page RW on ARMv6/v7

When emulating modified bit the executable attribute was cleared by
mistake when calling pmap_set_prot(). This was not a problem before
changes to ref/mod emulation since all the pages were created RW basing
on the "prot" argument in pmap_enter(). Now however not all pages are RW
and the RW permission can be cleared in the process.

Added proper KTRs accordingly.

Spotted by:	cognet
Reviewed by:	gber
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