New configuration option "RequireAuthPing": PING-PONG on login

When enabled, this configuration option lets ngIRCd send a PING with an
numeric "token" to clients logging in; and it will not become registered
in the network until the client responds with the correct PONG.

This is used by QuakeNet for example (ircu/snircd), and looks like this:

  NICK nick
  :irc.example.net PING :1858979527
  USER user . . :real name
  PONG 1858979527
  :irc.example.net 001 nick :Welcome to the Internet Relay Network ...