[RFA 00/67] Simple add_cmd constification

[Tom Tromey <tom at tromey dot com>]

First, left me offer my apologies for the length of this series.  It
was somewhat simpler to break it into many tiny pieces, because
sometimes when const-ifying it is easy to get in over one's head.  On
the plus side, nearly all of the patches are trivial and so review
should not be too difficult -- just boring.

This starts by adding const-correct overloads of add_cmd and a few
related functions.  Then, it converts many functions to use the
const-correct form.  The idea is to gradually convert all commands
(and set/show, etc) to const-correctness, then remove the non-const
overloads.

Regression tested on the buildbot.

Tom