Empty key arguments work fine when passed as command arguments, but
not interactively. For example, 'edit s 42 R ""' clears the retreat
path, but in an interactive 'edit s 43', 'R ""' sets it to "".
In Empire 1, omitting the argument made it empty. Empire 2 turned
that into an error without providing an alternative.
Switch to the common command parser, so that quoting works, and "" is
parsed as empty argument.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>