Make rcirc logging more customizable

Make rcirc logging more customizable

(rcirc-log-filename-function): New variable.
(rcirc-log): Use `rcirc-log-filename-function' to generate the
log-file name.  Don't log anything if it returns nil.
(rcirc-log-write): Use `expand-file-name' when merging the
log-file name from the alist with rcirc-log-directory; this does
the right thing if the name in the alist already an absolute
filename.  Make the log-file directory if necessary.

Revision: emacs@sv.gnu.org/emacs--devo--0--patch-976