Allow escaping of initial / in rcirc

[Björn Lindström]

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and the precise symptoms of the bug:

In correspondence to how this usually works in IRC clients, '//' at the
start of a line should translate to '/', rather than be interpreted as a
command.
In GNU Emacs 22.0.50.1 (i486-pc-linux-gnu)
 of 2005-12-21 on pacem, modified by Debian
configured using `configure '--build' 'i486-linux-gnu' '--host' 
'i486-linux-gnu' '--prefix=/usr' '--sharedstatedir=/var/lib' 
'--libexecdir=/usr/lib' '--localstatedir=/var' '--infodir=/usr/share/info' 
'--mandir=/usr/share/man' '--with-pop=yes' 
'--enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/22.0.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/22.0.50/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/22.0.50/leim'
 '--with-x=no' 'CFLAGS=-DDEBIAN -g -Wno-pointer-sign -O2' 
'build_alias=i486-linux-gnu' 'host_alias=i486-linux-gnu''

Major mode: rcirc