(nnmairix-group-regexp, nnmairix-valid-backends): Convert from free
variables to defconsts. Convert comments to doc-strings.
(nnmairix-last-server, nnmairix-current-server): Convert from free
variables to defvars. Convert comments to doc-strings.
(gnus-registry-fetch-group): Autoload.
(nnmairix-replace-group-and-numbers): Use mapc rather than mapcar.
(nnmairix-widget-get-values, nnmairix-widget-make-query-from-widgets)
(nnmairix-widget-build-editable-fields): Use car cddr rather than caddr.