Slowly start implementing tagging support for man(7) pages, even

Slowly start implementing tagging support for man(7) pages, even
though it is obvious that this can never become as good as for
mdoc(7) pages.  As a first step, tag alphabetic arguments of .IP
macros, which are often used for lists of options and keywords.

Try "man -O tag=g as" to get the point.

Thanks to Leah Neukirchen for recently reminding me that exploring
how much can be done in this respect may be worthwhile: it is likely
to slightly improve usability while adding only small amounts of
relatively straightforward code.