The new `url-user-agent' variable can be customized to be a string or
a function.
+---
*** The new interface variable `url-request-noninteractive' can be used
to specify that we're running in a noninteractive context, and that
we should not be queried about things like TLS certificate validity.
+---
*** If URL is used with a https connection, the first callback argument
plist will contain a :peer element that has the output of
`gnutls-peer-status' (if Emacs is built with GnuTLS support).