core: Handle single-parameter PONG replies

core: Handle single-parameter PONG replies

In processIrcEventPong, allow for single-parameter PONG replies.
Some IRC servers reply with server name then quote, others reply with
just the quote.

Log a debug message for invalid timestamps.  It's not a known error
as the user can request arbitrary PING replies, but it should be
logged to simplify tracking down IRC servers that mangle or outright
ignore the PING parameters.

(Yes, some IRC servers do this.)

Handle single-parameter PONG replies in EventStringifier, too.

Clarify and reformat comments.