HAMMER - Add additional warnings for v2/v3 upgrades, fix bug in snaprm

HAMMER - Add additional warnings for v2/v3 upgrades, fix bug in snaprm

* Add warnings related to backup scripts when upgrading from V2 to V3.
  The synctid directive does not generate a snapshot.  You should generate
  a real snapshot to properly track them.

  Also, removing a snapshot softlink does not remove the meta-data and
  thus the pruning code will continue to prune around that snapshot.
  The snaprm directive should be used when removing snapshot softlinks
  to also remove the meta-data.

* Snaprm now uses the current directory when removing a transaction id
  and no target directory was specified.

* Snaprm now makes it clear that a failure to remove meta-data when
  removing a softlink is just a warning, not a fatal error.