Add org.freedesktop.UDisks2.Swapspace interface

Add org.freedesktop.UDisks2.Swapspace interface

Whether an swap device is in use or not, can be tracked via the
:Active property. A swap device can be activated or deactivated with
the Start() and Stop() methods.

For now, Start() and Stop() are not actually hooked up.

Implementation-wise, monitoring /proc/swaps requires this patch

 http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=66d7dd518ae413a383ab2c6c263cc30617329842

which is in Linux 2.6.38 or later.

Example:

 $ udisksctl monitor
 Monitoring the udisks daemon. Press Ctrl+C to exit.
 13:49:08.441: The udisks-daemon is running (name-owner :1.315).
 13:49:11.720: /org/freedesktop/UDisks2/block_devices/sda3: org.freedesktop.UDisks2.Swapspace: Properties Changed
   Active:               false
 13:49:11.746: /org/freedesktop/UDisks2/block_devices/sda3: org.freedesktop.UDisks2.LinuxSysfsDevice::UEvent (b'change',)
 13:49:15.310: /org/freedesktop/UDisks2/block_devices/sda3: org.freedesktop.UDisks2.Swapspace: Properties Changed
   Active:               true

Signed-off-by: David Zeuthen <davidz@redhat.com>