sysexits.3: Enable apropos by error number

[ziaee]

Summary

Markup exit status codes as defined variables. This produces no change
in rendered output, however enables this workflow:

$ apropos Dv=67
sysexits(3) - legacy exit status codes for system programs

MFC after: 3 days

[mhorne]

We can quickly enter bikeshed territory debating something like this, but I must ask, is this workflow real or imagined?

It is somewhat of a stretch to declare numeric literals as Dv, that is "defined variable or preprocessor constant"; pedantically they are neither. Of course I favor practicality over pedantics.

Why does the change not add Dv markup for the names of the constants? Is the user any more or less likely to search apropos Dv=EX_USAGE?

I can mostly grasp the rationale of the change, but I think you could do a little more to convince the reader of its value.