Add backing filesystem info to docker info command where applicable

Fixes #9960

This adds the output of a "Backing Filesystem:" entry to `docker info`
to overlay, aufs, and devicemapper graphdrivers. The default list
includes a fairly complete list of common filesystem names from
linux/include/uapi/linux/magic.h, but if the backing filesystem is not
recognized, the code will simply show "<unknown>"

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>
Upstream-commit: 74b009e816
Component: cli

components/cli/docs/sources/reference/commandline/cli.md

@@ -1171,6 +1171,7 @@ For example:
     Images: 52
     Storage Driver: aufs
      Root Dir: /var/lib/docker/aufs
+     Backing Filesystem: extfs
      Dirs: 545
     Execution Driver: native-0.2
     Kernel Version: 3.13.0-24-generic