eclass/savedconfig.eclass: rewrite the ROFF macros

eclass/savedconfig.eclass: rewrite the ROFF macros

man2html cannot cope with multi-character number-register names and
also doesn't recognize the square brackets in the '\n' escape
sequence. This breaks the autogenerated HTML output. Rename the
number-register from "step" to "R" and get rid of the square brackets.

URL: https://archives.gentoo.org/gentoo-dev/message/b619a5775cbe6d0b650d827338ef8ca2
Closes: https://bugs.gentoo.org/699476

Signed-off-by: Göktürk Yüksek <gokturk@gentoo.org>
Acked-by: Lars Wendler <polynomial-c@gentoo.org>