[RFA 0/4] finish command constification

[Tom Tromey <tom@tromey.com>]

This series completes my command constification efforts.  This turned
out to be much simpler than I had anticipated; partly because I
started automating some of the changes, and partly because C++ makes
some incremental refactorings simpler -- as discussed before, in this
case by allowing the introduction of const overloads.

This series touches some files I can't compile.  I made a best effort
to inspect them for const correctness; but it's hard to be sure.
Given the experience of this series as a whole, I think it is safe to
say that any required changes will be minimal.

These patches point out that a number of spots in gdb were not really
const-correct.  For example there was code like:

    some_command ((char *) "a constant"), ...)

These weren't actually unsafe but nevertheless were ugly.

Regression tested by the buildbot.

Tom