Stop using PVF_MOD, PVF_REF & PVF_EXEC flags in pv_entry, use PTE.

Using PVF_MOD, PVF_REF and PVF_EXEC is redundant as we can get the proper
info from PTE bits.
When the mapping is marked as executable and has been referenced we assume
that it has been executed. Similarly, when the mapping is set to be writable
and is referenced, it must have been due to write access to it.
PVF_MOD and PVF_REF flags are kept just for pmap_clearbit() usage,
to pass the information on which bit should be cleared.

Submitted by:   Zbigniew Bodek <zbb@semihalf.com>
Sponsored by:   The FreeBSD Foundation, Semihalf
Rework and organize pmap_enter_locked() function.

pmap_enter_locked() implementation was very ambiguous and confusing.
Rearrange it so that each part of the mapping creation is separated.
Avoid walking through the redundant conditions.
Extract vector_page specific PTE setup from normal PTE setting.

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