Use minus signs instead of hyphens where appropriate

Command-line options use minus signs, but groff interprets '-' as a
hyphen, which is a different Unicode character. For text which may be
copied to a command-line for re-use, it's preferable to use explicit
minus signs, '\-' in groff. See
http://lintian.debian.org/tags/hyphen-used-as-minus-sign.html for more
details.