Proposed: treat color, cp, linetabs, vpt request arguments as *roff Booleans

[G. Branden Robinson]

Hi all,

I have another language change proposal.  I suspect impact will be low,
but it is strictly speaking a behavior change.

o The `color`, `cp`, `linetabs`, and `vpt` requests now interpret
  arguments with negative values as instructions to disable the
  corresponding feature, using the *roff integer-to-Boolean conversion
  idiom instead of the C/C++ one.  Thus, if you invoke these requests
  with a register interpolation, the outcome agrees with an `if` test of
  the register's value.

Here is a motivating example.

.nr i-want-compatibility-mode -1
.if \n[i-want-compatibility-mode] .tm I want compatibility mode!
.cp \n[i-want-compatibility-mode]
.ie \n(.C .tm I'm in compatibility mode now.
.el       .tm I am NOT in compatibility mode.

groff 1.22.4, 1.23.0, and Git HEAD:

I'm in compatibility mode now.

groff with this patch:

I am NOT in compatibility mode.

Patch attached.

Regards,
Branden

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