This moves supportsMultipleLowerDir() to overlayutils
so it can be used from both overlay and overlay2.

The only changes made were:
 * replace logger with logrus
 * don't use workDirName mergedDirName constants
 * add mnt var to improve readability a bit

This is a preparation for the next commit.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit d568707)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Before this commit, overlay check was performed by looking for
`overlay` in /proc/filesystem. This obviously might not work
for rootless Docker (fs is there, but one can't use it as non-root).

This commit changes the check to perform the actual mount, by reusing
the code previously written to check for multiple lower dirs support.

The old check is removed from both drivers, as well as the additional
check for the multiple lower dirs support in overlay2 since it's now
a part of the main check.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 649e4c8)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Now that we do check if overlay is working by performing an actual
overlayfs mount, there's no need in extra checks for the kernel version
or the filesystem type. Actual mount check is sufficient.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit e226aea)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>