[m-rev.] for review: options to control the use of color

[Zoltan Somogyi]

For review by anyone.

The use of colors is still off by default. It can now be switched on
by either --enable-color-diagnostic, or by --config-default-color-diagnostics,
after which the latter of which it can also be switched off by --no-enable-
color-diagnostics. The -set-color-* options also work.

Once we agree on the set of options, including their names and intended effects,
I will update doc/user_guide.texi with the (for now commented-out) option
descriptions.

Zoltan.
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