swap: use swapon -o

This patch simplify swapon usage in systemd. The command swapon(8)
since util-linux v2.26 supports "-o <list>". The idea is exactly the
same like for mount(8). The -o specifies options in fstab-compatible
way. For systemd it means that it does not have to care about things
like "discard" or another swapon specific options.

        swapon -o <options-from-fstab>

For backward compatibility the code cares about "Priority:" swap unit
field (for a case when Priority: is set, but pri= in the Options: is
missing).

References: http://lists.freedesktop.org/archives/systemd-devel/2014-October/023576.html

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