emerge: protect symlinks to directories sometimes

emerge: protect symlinks to directories sometimes

Before, it was possible to unmerge a symlink to a directory, such that
files installed via the path of the symlink could become inaccessible
via that path (and also making it impossible to unmerge them via that
path).

Now, the symlink will only be unmerged if the directory that it points
to only contains regular files which are all being unmerged. In any
other case, the symlink will be preserved and an eerror log message
will record the event. This will give the user an opportunity to take
further action if they deem it necessary, and such symlink preservation
will not be silent as it was reported in bug #326685, comment #3.