man: Syntax and spelling fixes.

Signed-off-by: Ville Skyttä <ville.skytta@iki.fi>
losetup, mount: remove --pass-fd at all

The option is unsupported since v2.23, let's cleanup man pages and
code to remove it at all.

Reported-by: "Dale R. Worley" <worley@alum.mit.edu>
Signed-off-by: Karel Zak <kzak@redhat.com>