docs: fix inconsistencies in man pages

Signed-off-by: Karel Zak <kzak@redhat.com>
zramctl: remove extra \n from usage()

Signed-off-by: Karel Zak <kzak@redhat.com>
cfdisk: move curs_set(1) to ui_end()

This is probably correct to have curs_set() in ui_end(), but according
to Martin it does not work with slang library from signal handler.

Reported-by: Martin Steigerwald <Martin@lichtvoll.de>
Signed-off-by: Karel Zak <kzak@redhat.com>
swapon: remove invalid error string print outs

To avoid messages like this:

$ swapon /dev/zram0
swapon: /dev/zram0: read swap header failed: Success

Signed-off-by: Sami Kerola <kerolasa@iki.fi>