Medium: symlink: Handle missing directories in target (bsc#973054)

If the directory in which the symlink target is not existing, migration
after failure fails due to the probe reporting an error on the failed
node with a broken symlink.

To handle this, use "readlink -m" over "readlink -f". From the readlink
man page:

-f, --canonicalize
       canonicalize by following every symlink in every component of the given name recursively; all but the last component must exist

-e, --canonicalize-existing
       canonicalize by following every symlink in every component of the given name recursively, all components must exist

-m, --canonicalize-missing
       canonicalize by following every symlink in every component of the given name recursively, without requirements on components existence