findmnt: add SOURCES column to print all devices with the same tag

findmnt: add SOURCES column to print all devices with the same tag

It's the same like TARGETS for lsblk (the same device is possible to
mount on more mountpoints). Here in findmnt we support by a new column
SOURCES scenario when more devices (filesystems) use the same tag
(LABEL, UUID, e.g.).

For example:

$ findmnt --fstab -oTARGET,SOURCES --evaluate --json UUID=b45897ca-ffc4-4089-8404-5d6072f5aee1
{
   "filesystems": [
      {
         "target": "/mnt/test",
         "sources": [
             "/dev/sdb1", "/dev/sdc"
         ]
      }
   ]
}

Fixes: https://github.com/util-linux/util-linux/issues/1078
Signed-off-by: Karel Zak <kzak@redhat.com>