swapon: add -o <list> for compatibility with mount

The new option allows to specify swap options by fstab compatible
string. The concept is the same as for mount(8).

   swapon -o pri=1,discard=pages,nofail /dev/sda2

The advantage is that tools (like systmed) that parses fstab can call
swapon without translation from fstab options to swapon(8) command
line options.

Signed-off-by: Karel Zak <kzak@redhat.com>