allow activating known LVs when other LVs have unknown segtypes

allow activating known LVs when other LVs have unknown segtypes

When a VG contains some LVs with unknown segtypes, the user
should still be allowed to activate other LVs in the VG that
are understood.

$ lvs foo
  WARNING: Unrecognised flag CACHE_USES_CACHEVOL in segment type cache+CACHE_USES_CACHEVOL.
  WARNING: Unrecognised segment type cache+CACHE_USES_CACHEVOL
  LV    VG  Attr       LSize
  lvol0 foo -wi-------  4.00m
  other foo vwi---u--- 48.00m

$ lvcreate -l1 foo
  WARNING: Unrecognised flag CACHE_USES_CACHEVOL in segment type cache+CACHE_USES_CACHEVOL.
  WARNING: Unrecognised segment type cache+CACHE_USES_CACHEVOL
  Cannot change VG foo with unknown segments in it!
  Cannot process volume group foo

$ lvchange -ay foo/lvol0
  WARNING: Unrecognised flag CACHE_USES_CACHEVOL in segment type cache+CACHE_USES_CACHEVOL.
  WARNING: Unrecognised segment type cache+CACHE_USES_CACHEVOL

$ lvchange -ay foo/other
  WARNING: Unrecognised flag CACHE_USES_CACHEVOL in segment type cache+CACHE_USES_CACHEVOL.
  WARNING: Unrecognised segment type cache+CACHE_USES_CACHEVOL
  Refusing activation of LV foo/other containing an unrecognised segment.

$ lvs foo
  WARNING: Unrecognised flag CACHE_USES_CACHEVOL in segment type cache+CACHE_USES_CACHEVOL.
  WARNING: Unrecognised segment type cache+CACHE_USES_CACHEVOL
  LV    VG  Attr       LSize
  lvol0 foo -wi-a-----  4.00m
  other foo vwi---u--- 48.00m