dist-docs: Fix text and HTML manpage generation with some groff versions.

dist-docs: Fix text and HTML manpage generation with some groff versions.

Some versions of groff use termcap sequences for bold, italic, etc. by
default.  The dist-docs script doesn't cope with those; it expects
sequences based on backspacing and overprinting.  This commit fixes the
problem by setting an environment variable GROFF_NO_SGR that forces groff
to use backspacing.

Found on Fedora.

Reported-by: Russell Bryant <rbryant@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <rbryant@redhat.com>