doc/extban: Correct description of EXTBAN token in 005.

The change from EXTBAN=$:<types> to EXTBAN=$,<types> was made later on
and the documentation was not adjusted.
listener: Move error messages to something visible by default.

If a listening port cannot be opened, send error messages to opers with
snomask +s and ircd.log, instead of snomask +d and the ioerror log, which
both are usually disabled.

Also, restore information about what listener is having problems. This
was lost when report_error() was replaced.