varlinkctl: add new list-methods verb

varlinkctl: add new list-methods verb

For putting together "varlinkctl call" command lines it's useful to
quickly enumerate all methods implemented by a service. Hence, let's add
a new "list-methods" which uses the introspection data of a service to
quickly list methods.

This is implemented as a special flavour of the "introspect" logic,
and just suppresses all output except for the method names.