lvconvert --merge @tag support

lvconvert --merge @tag support

Switch lvconvert's --merge code over to using process_each_lv().  Doing
so adds support for a single 'lvconvert --merge' to start merging
multiple LVs (which includes @tag expansion).

Add 'lvconvert --merge @tag' testing to test/t-snapshot-merge.sh

Adjust man/lvconvert.8.in to reflect these expanded capabilities.

The lvconvert.c implementation requires rereading the VG each iteration
of process_each_lv().  Otherwise a stale VG instance associated with
the LV passed to lvconvert_single_merge() would result in stale VG
metadata being written back out to disk.  This overwrote new metadata
that was written when a previous snapshot LV finished merging (via
lvconvert_poll).  This is only an issue when merging multiple LVs that
share the same VG (a single VG is typical for most LVM configurations on
system disks).

In the end this new support is very useful for performing a "system
rollback" that requires multiple snapshot LVs be merged to their
respective origin LV.

The yum-utils 'fs-snapshot' plugin tags all snapshot LVs that it creates
with a common 'snapshot_tag' that is unique to the yum transaction.
Rolling back a yum transaction, that created LVM snapshots with the tag
'yum_20100129133223', is as simple as:
  lvconvert --merge @yum_20100129133223

Signed-off-by: Mike Snitzer <snitzer@redhat.com>