nspawn: tweaks to /etc/resolv.conf management

Handle properly if /etc is a symlink (i.e. make sure we don't follow the
symlink outside the image). Also follow /etc/resolv.conf if it is a
symlink, and use the resolved path when creating a mount point and
mounting (as both of these operations follow symlinks and rally
shouldn't).

Handle more types of read-only errors as debug-level issues.