When rendering the \h (horizontal motion) low-level roff(7) escape

When rendering the \h (horizontal motion) low-level roff(7) escape
sequence in -T ps and -T pdf output mode, use an appropriate
horizontal distance by correctly using the term_len() utility
function.  Output from the -T ascii, -T utf8, and -T html modes
was already correct and remains unchanged.

Lennart Jablonka <hummsmith42 at gmail dot com> found and reported
this unit conversion bug (misinterpreting AFM units as if they were
en units) when rendering scdoc-generated manuals (which is a low
quality generator, but that's no excuse for mandoc misformatting \h)
on Alpine Linux.  Lennart also tested this patch.