Handle unusual case of missing mdadm but SWRaid arrays active (#756829)

If the installation is unusual / broken such that the mdadm command is
not found but there are active SWRaid arrays, provide a fallback with
some of the information.  In this case populate the cache with details
only available from /proc/mdstat.  Information will be limited to active
arrays only and their members.  No UUIDs or labels.  There will be no
information about inactive arrays and GParted will use it's normal
libparted and blkid identification for those devices and partitions.

As mdadm has gained the capability to manage Fake/BIOS RAID arrays they
also appear in /proc/mdstat when mdadm is used to start them.  Enhance
the parser of /proc/mdstat to only extract information for SWRaid arrays
with recognised metadata versions.

Bug 756829 - SWRaid member detection enhancements