arm: Unify naming in pmap-v6.c and improve style

Use naming common for whole pmap and compatible with other pmaps:
pm   -> pmap
pg   -> m
opg  -> om
*pt  -> *ptep
*pte -> *ptep
*pde -> *pdep

Improve style where possible.
arm: Get rid of VERBOSE_INIT_ARM option

This "NetBSDism" won't actually work on FreeBSD as the console is being
initialized after pmap preliminary setup.
On the other hand, drivers that were using VERBOSE_INIT_ARM for debug purposes
are not part of or are not called in initarm() so the option is misused.
In such cases exchange VERBOSE_INIT_ARM to debug printf (DPRINTF) and delete
all other useless VERBOSE_INIT_ARM ifdefs.
arm: Avoid possible NULL pointer dereference in pmap_enter_locked()

When trying to get m->md.pv_memattr, make sure that m != NULL,
in particular that vector_page is set to be NULL.
arm: Do not set PGA_REFERENCED flag in pmap_enter_pv()

On ARM any new page reference will result in either entering the new mapping by
calling pmap_enter, etc. or fixing-up the existing mapping in
pmap_fault_fixup().
Therefore we set PGA_REFERENCED flag in the earlier mentioned cases and
setting it later in pmap_enter_pv() is just waste of cycles.
arm: Stop using PVF_MOD, PVF_REF & PVF_EXEC flags in pv_entry. Use PTE instead.

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
is 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 about which bit should be cleared.