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.

Conflicts:
	src/net.cpp
move GetLegacySigOpCount and GetP2SHSigOpCount out of CTransaction

Conflicts:
	src/main.h