[RFA 0/3] preparation for command constification; cleanup removal

[Tom Tromey]

In addition to the removal of cleanups, I've also long wanted to clean
up gdb by changing commands to take a "const char *".

With C++ this is simpler to do now, because we can use overloading to
minimize the neede code changes.  To that end, the first patch in this
series renames some functions so we can take advantage of overloading
in the future.

The second patch is a minor constification patch in preparation for
the third patch.

The third patch changes extract_arg to return a std::string, which
allows for some cleanup removal.

Regression tested by the buildbot.

Tom