Add dm_pool_strdup to allocate memory and copy a tag in {lv|vg}_change_tag()

Add dm_pool_strdup to allocate memory and copy a tag in {lv|vg}_change_tag()

We need to allocate memory for the tag and copy the tag value before we
add it to the list of tags.  We could put this inside lvm2app since the
tools keep their memory around until vg_write/vg_commit is called, but
we put it inside the internal library to minimize code in lvm2app.
We need to copy the tag passed in by the caller to ensure the lifetime of
the memory until the {vg|lv} handle is released.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>