There _is_ an underlying logic to these commands, but it isn't
wholly uniform, given years of tweaks and changes.  Fortunately I
think there is a superset that will work.

This commit adds a parser for some of the most basic cases -- the
ones currently handled by getargs_helper() and some of the
object-taking ones.  Soon will come initial tests; then I'll start using
the parser.

After that, I'll expand the parser to handle the other cases that come
up in the controller protocol.