Refactor -alertnotify code

Refactor common -alertnotify code into static CAlert::Notify method.
Make -proxy set all network types, avoiding a connect leak.

Previously -proxy was not setting the proxy for IsLimited networks, so
 if you set your configuration to be onlynet=tor you wouldn't get an
 IPv4 proxy set.

The payment protocol gets its proxy configuration from the IPv4 proxy,
 and so it would experience a connection leak.

This addresses issue #5355 and also clears up a cosmetic bug where
 getinfo proxy output shows nothing when onlynet=tor is set.

Conflicts:
	src/init.cpp

Rebased-From: 3c777141349ad82d679a278df0619968af53c23
Github-Issue: #5358