Reduce instances of `` '' in manuals.

troff displays these as typographic quotes, but nroff implementations
almost always print them literally, which rarely has the intended effect
with modern fonts, even in stock xterm.

These uses of `` '' can be replaced either with more semantic alternatives
or with Dq, which prints typographic quotes in a UTF-8 locale (but will
automatically fall back to `` '' in an ASCII locale).

improvements and ok schwarze@