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>