node, cmd/clef: report actual port used for http rpc

[holiman]

This PR fixes and issue where, if you specify --rpcport=0, you get

HTTP endpoint opened                     url=http://127.0.0.1:0 cors= vhosts=localhost

Port 0 means "hey OS, please give me a free port". We should output the actual
port given to us by the OS in this case.

With this PR:

geth

INFO [03-20|10:11:00.749] IPC endpoint opened                      url=/tmp/geth.ipc
INFO [03-20|10:11:00.749] HTTP endpoint opened                     url=http://127.0.0.1:43157/ cors= vhosts=localhost
...
INFO [03-20|10:11:02.546] HTTP endpoint closed                     url=http://127.0.0.1:43157
INFO [03-20|10:11:02.546] IPC endpoint closed                      url=/tmp/geth.ipc

clef

DEBUG[03-20|10:10:16.333] HTTP registered                          namespace=account
INFO [03-20|10:10:16.333] HTTP endpoint opened                     url=http://127.0.0.1:39755
...
INFO [03-20|10:10:19.130] IPC endpoint closed                      url=/home/user/.clef/clef.ipc
INFO [03-20|10:10:19.130] HTTP endpoint closed                     url=http://127.0.0.1:39755

[karalabe]

Is the trailing / accidental or deliberate?

[holiman]

accidental... :/

[holiman]

No wait, it's actually correct to use a trailing slash there. I think it's the more correct URL, since it's a GET / HTTP/1,1 -- a domain is not a valid url