scripts: use build_ prefix for string not piped through cgen()

The gen_ prefix is awkward.  Generated C should go through cgen()
exactly once (see commit 1f9a7a1a).  The common way to get this wrong is
passing a foo=gen_foo() keyword argument to mcgen().  I'd like us to
adopt a naming convention where gen_ means "something that's been piped
through cgen(), and thus must not be passed to cgen() or mcgen()".
Requires renaming gen_params(), gen_marshal_proto() and
gen_event_send_proto().

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170601124143.10915-1-marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>