Added AuthViaIpCheckBox which allows authentication (for relay) via POP3
or other protocol within the last 60 minutes (this was previously always
enabled if authenticated-relay was allowed, but now that we have real SMTP
AUTH support, this less-secure method is optional).

Created system.lastuser property to get last user record number.
Updated property and method descriptions.
system.secondstr() returns null if no argument specified.

before common global functions).
Updated global method descriptions.

before common global functions).

(skipping deleted or inactive user records).

leaving last_error alias for backwards compatibility.

POP3 server thread reads all msg indices and headers before reporting
initial OK or ERR reponse after authentication.