Don't poll showmyip.com, it doesn't exist anymore

Fixes #4679.

This leaves us with only one candidate, checkip.dyndns.org.
GetMyExternalIP should be phased out as soon as possible.

Rebased-From: c33b983
Avoid querying DNS seeds, if we have open connections.

The goal is to increase independence and privacy.
gitian: Bump miniupnp version to 1.9.20140701

Also change build system: STATICLIB is now MINIUPNP_STATICLIB.
ExtractAddress() -> ExtractDestination()

Fixed comments
 New issue Don't reveal whether password is <20 or >20 characters in RPC

From Bitcoin Pull #4728

It seems bad to base a decision to delay on the password length, as it leaks a tiny bit of information.

This doesn't change DoS potential as it is trivial to hold up all RPC threads in another way for someone in the rpcallowip list.
Merge pull request #689 from whitj00/patch-4

 New issue Don't reveal whether password is <20 or >20 characters in RPC
Merge pull request #690 from whitj00/patch-3

ExtractAddress() -> ExtractDestination()
Merge pull request #674 from rnicoll/1.8.1-dev-networking

Apply network related changes from 0.9.3
Merge pull request #678 from rnicoll/1.8.1-dev-dependencies2

gitian: Bump miniupnp version to 1.9.20140701