Defparser
Register a new argument-parsing function with getopt.
(defparser fn &key predicate) is a macro for registering parsing functions with
getopt.
It first checks to make sure that fn has the valid format for a custom-parser, and tries to prove the necessary progress property.
If predicate is non-nil, it installs fn as the default parsing
function to use for options of type predicate. The general idea here is
that if you write a new custom parser for ports, you can then set it up to be
the default parser for any port-p field.
