[MAN] Markup fixes in manual for mandoc 1.12.1

with Aleksey we briefly discussed the mdoc compatibility of the manpage,
here's a patch that makes mandoc 1.12.1 happier and behaves correctly
against groff 1.21.  I want to include it in the staging dash-0.5.7
OpenBSD port.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>