lockd: allow nolocking and readonly options

lockd: allow nolocking and readonly options

When --nolocking is used (by vgs, lvs, pvs):

. don't use lvmlockd at all (set use_lvmlockd to 0)
. allow lockd VGs to be read

When --readonly is used (by vgs, lvs, pvs, vgdisplay, lvdisplay,
pvdisplay, lvmdiskscan, lvscan, pvscan, vgcfgbackup):

. skip actual lvmlockd locking calls
. allow lockd VGs to be read
. check that only shared gl/vg locks are being requested
  (even though the actually locking is being skipped)
. check that no LV locks are requested, because no LVs
  should be activated or used in readonly mode
. disable using lvmetad so VGs are read from disk

It is important to note the limited commands that accept
the --nolocking and --readonly options, i.e. no commands
that change/write a VG or change/activate LVs accept these
options, only commands that read VGs.