ircs:// integration for rcirc (bug#56514)

Hi Philip,

Just a heads up: I'm in the process of possibly tweaking url-irc (and
browse-url) to better support irc:// links in internal Emacs apps. If
you're interested, this mostly concerns the second patch here:

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=56514#22

Basically, I wanted a way to tell `url-irc-erc' whether to connect via
TLS without relying on port numbers and without changing the signature
of `url-irc-function'. Rather than mess with `func-arity' or the like,
I've opted to just hijack an existing variable from the url(-vars)
library, `url-current-object', which seems ready made for this purpose.
The idea is to simply bind it to the parsed URL object during calls to
`url-irc-function'. Please let me know if you see any downsides
to doing this or if a smarter approach comes to mind.

BTW, as the patch shows, I've left the rcirc side alone. But if you want
to handle ircs:// links without bothering with all the
`url-current-object' business above, simply arranging to switch to 'tls
whenever the port is 6697 should have you pretty well covered, I think.
Let me know if that doesn't make sense.

Thanks,
J.P.