svnmucc should grow symlink operation

The Editor API goes in the direction of represention symlinks explicitly. 
Accordingly, we should allow

% svnmucc -mm -U $URL symlink linkname target

instead of the cumbersome and full-of-implementation-detail

% svnmucc -mm -U $URL \
    put =(printf "link %s" "target") \
    linkname propset svn:special '*' linkname