man pages: fix some formatting

man pages: fix some formatting

I noticed one place where -- was used instead of \--, leading to a
emdash instead of two hyphens, but then I looked and realized this
was not just a single place; fix all of those.

While at it, I saw some --keep options being referenced without
backticks, fix that too.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
(cherry picked from commit 27e6ee3680585fd445ce04955333a4bdf585db30)
Tested-by: Rob Browning <rlb@defaultvalue.org>