swapon: rewrite control struct usage

swapon: rewrite control struct usage

The old version has been pretty broken... the most important is to
keep swap options specified on command line as read-only template.
For example if we call "swapon --all" then we cannot modify the global
options for each fstab swap entry.

The another story has been control struct modification due to device
reinitialization etc.

This patch splits all to:

* struct swapon_control; top-level struct with command line options

* struct swap_device; this is device specific and never globally
  maintained by swapon_control.

* struct swap_prop; used as global read-only template swap options
  and per device swap options (when parse fstab).

Addresses: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=818252
Signed-off-by: Karel Zak <kzak@redhat.com>