[bug #64438] [ms] validation of `PS` macro arguments too strict

[G. Branden Robinson]

Follow-up Comment #1, bug #64438 (project groff):

It doesn't appear that I forgot anything.  Doug is using a vaguely documented
interface, apparently one of long pedigree.

CSTR #116 (May 1991 revision) says on p. 18:

> _Pic_ copies the .PS and .PE lines from input to output intact,
> except that it adds two things on the same line as the .PS:
>
> .PS h w
> 
> h and w are the picture height and width in units.  The -ms macro
> package has definitions for .PS and .PE that cause pictures to be
> centered and offset a bit from surrounding text.  (See the
> appendix).

(Does "adds" mean "prefixes" or "suffixes" [existing arguments]?  "Prefixes"
is what is actually done.)

Consulting the appendix on p. 26, we see:

> This is the default definition of the .PS and .PE macros:
>
> .de PS \" start picture; $1 is height, $2 is width, in inches
> .sp .3
> .in (\\n(.lu-\\$2)/2u
> .ne \\$1
> ..
> .de PE \" end of picture
> .in
> .sp .6
> ..

I'm pretty sure this is what I went by when updating the `@PS` macro
definition in _groff ms_.

Permit me to register my desire to send a Daffy Duck tantrum about this
imprecision back to Murray Hill in a DeLorean^W^Wthe 1980s.

Anyway, I'll fix _groff ms_ and add some documentation to our _pic_(1) page
documenting this detail (that arguments to `PS` in the input are shifted, and
their interpretation is the province of the macro definition).


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