[Nmh-workers] .BR and .IR for program names in man pages

[Larry Hynes]

There's a bit a mix of .BR and .IR used in the man pages, specifically
when referring to another program, as in

        .BR dist ,

or

        .IR mh-draft (5)

My (limited) understanding is that the 'correct' approach is to
always use .BR in such cases; or, to put it another way, names of
programs should be bold, and options should be italic.

This is something I can clean up 'as I go', if there's consensus
on the desired approach.