MFC: following 4 commits:

MFC: following 4 commits:
undo(1) doc fixes:
   - sync usage() to undo.1
   - improve wording a bit in a few places

Update `-t' flag; now it is always transaction-id.

Add logic to warn of possible renames, and clearly state when failures may
  be due to prior renames.

  Change -h to -a (-a for 'all'), which is a more obvious option name.

Add missing space and newline to error message.