Support for name lookups in -connect and -addnode

• A new option -dns is introduced that enables name lookups in -connect and -addnode, which is not enabled by default, as it may be considered a security issue.
• A Lookup function is added that supports retrieving one or more addresses based on a host name
• CAddress constructors (optionally) support name lookups.
• The different places in the source code that did name lookups are refactored to use NameLookup or CAddress instead (dns seeding, irc server lookup, getexternalip, ...).
• Removed ToStringLog() from CAddress, and switched to ToString(), since it was empty.