pre-revprop-change hooks

• block_bad_revprop_names.py: prevent adding rev properties with bad names.

• check_gvn_revprops.py: allow gvn to add gvn:approve:submitter and gvn:submitted, gvn:change, but no subsequent changes (users cannot delete their approval tracks).

• check_svn_revprops.py: block anything related to a svn:* rev property (including log entries).