The new `dns-get-srv` returns a *list* of srv-rr structs, rather than
zero or one. A general-purpose RR-lookup routine, `dns-lookup`, exists
but is not exported for now since its API needs work. See comments in
net/dns.rkt.
Add an optional #:browser argument to send-url/mac and export it.
This makes it straightforward to use different browsers and/or to
configure a different one:
(require net/sendurl)
(external-browser (lambda (url) (send-url/mac url #:browser "Conkeror")))
