FIX: Various Minor Bugs and UX Quirks in New Tag Implementation

This fixes various minor things we found yesterday in an interactive test session:

* deprecate `cvmfs_server list-tags` in favour of `cvmfs_server tag [-l]`
* properly check repository existence in `cvmfs_server tag`
* properly handle quotation marks in tag descriptions
* disallow tagging of nested catalogs
* don't close a repository transaction twice
* print deleted tags along with their root hashes after rollback
* disallow user fiddling with undo-tag (*trunk* and *trunk-previous*)

Most of those changes are also reflected in the test suite.