Revise L2 PTE access protections mask to consist of all permissions bits

L2_S_PROT_MASK did not include all the permissions bits.
Now it consists of L2_S_PROT_U (L2_AP[1]), L2_S_PROT_R (0)
L2_APX and L2_XN.
Notice that clearing those bits does not always take away the
corresponding permissions (in example the permission is granted
when the bit is cleared). The bits are cleared but are to be
set or left cleared accordingly in pmap_set_prot(), pmap_enter_locked(),
etc.

Submitted by:   Zbigniew Bodek <zbb@semihalf.com>
Sponsored by:   The FreeBSD Foundation, Semihalf