populate-volatile.sh: improve the handling of link config items
Previously, if there's a link config item in the config file like
l root root 1777 /tmp /var/tmp
and /tmp has existed, the symlink will not be created correctly.
Another example is the /run directory. If /run directory has been
created by some recipe or script before populate-volatile.sh runs,
the symlink of /run to /var/run will not be created correctly.
This patch ensures that the system creates symlinks exactly as the
config file tells it.
