Add reporting of linux-swap usage (#708107)

For active swap space read the usage from /proc/swaps.  (Linux kernel
uses units of 1 KiB).  By definition inactive swap space is 100% free.

    $ cat /proc/swaps
    Filename                                Type            Size    Used    Priority
    /dev/sda2                               partition       5242876 430552  -1

Always set fs.read = FS::EXTERNAL even if /proc/swaps doesn't exist so
that an attempt is made to open the file generating a specific error, in
addition to the generic error.

    open("/proc/swaps", O_RDONLY): No such file or directory

    Unable to read the contents of this file system!
    Because of this some operations may be unavailable.

    The cause might be a missing software package.
    The following list of software packages is required for linux-
    swap file system support: util-linux.

Closes Bug #708107 - Usage of swap space is not reported
Actually display error message from statvfs() failure

Pass error_message parameter by reference to
get_mounted_filesystem_usage() so that the caller can access the
modified error message when required.
Allow GParted to read the UUID from older btrfs-show command

All the code was there in btrfs::read_uuid() to read the UUID from the
btrfs-show command.  It just needed enabling.

This is only relevant when: (1) the blkid command is unavailable as
GParted primarily reads the UUID through the FS_Info cache using the
blkid command, and (2) the btrfs command is unavailable and the btrfs
module uses the older btrfs-show command instead.