Merge from erc--main--0

Merge from erc--main--0

Highlights:

 - Display messages starting with "/" but are not commands correctly.
 - Add erc-capab.el file, with capab-identify module.
 - Fix various ERC menu bugs and add "Current channel" submenu.
 - Make header-line display look nicer.
 - Fix warning when auto-authenticating to bitlbee.
 - New option erc-system-name allows the hostname next to your nick to be
   customized.

Revision: emacs@sv.gnu.org/emacs--devo--0--patch-599
Creator:  Michael Olson <mwolson@gnu.org>