man pages: replace all unescaped hyphen/minus characters with \-

man pages: replace all unescaped hyphen/minus characters with \-

In a man page, "-" officially means a typographical (Unicode) hyphen,
which frequently breaks the ability to copy and paste code examples from
a man page. "\-" means the ASCII hyphen/minus character. See
<http://lintian.debian.org/tags/hyphen-used-as-minus-sign.html> for
more details.

Rather than trying to distinguish between hyphens, em-dashes and
hyphen/minus, I just replaced all ambiguous hyphens with \- by applying
this vim command repeatedly until it didn't find anything:

     %s/\(^\|[^\\]\)-/\1\\-/g

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=38284
Reviewed-by: Lennart Poettering <lennart@poettering.net>