[RFA 0/8] Constify many commands

[Tom Tromey <tom at tromey dot com>]

This series changes add_abbrev_prefix_cmd,
add_com_suppress_notification, and add_com to take
cmd_const_cfunc_ftype rather than cmd_cfunc_ttype; then cleans up all
the users.

Some cleanups are removed as well, because changes were already needed
in some areas where moving to the cleanup-free style was more
convenient.

This probably overlaps a bit with some of my other breakpoint patches.
Those are still waiting for feedback.

Most of the patches are straightforward.

Patch #7 adds a new function to deal with how "list" and "x" work.

Patch #8 removes some non-const overloads that were added in a
previous series; but also adds some new ones.  The eventual conversion
of add_info should remove these.

Regression tested by the buildbot.  I also built (but did not regtest)
each patch (that is, each spot in the series at least compiles).

Tom