merge: handle symlink replacing dir

merge: handle symlink replacing dir

Previously, these symlinks would trigger a file collision message and
would then be silently dropped in cases when the file collsion did not
cause emerge to bail out due to FEATURES=protect-owned or
collision-protect.

Now, if emerge doesn't bail out due to a file collision, it will
trigger an eerror message and merge the symlink with a .backup.XXXX
extension appended to the file name, similar to handling of regular
files in commit 740f71301ed3daf44c0e77df5d5de39fe1438fb1. This will
fix bug 326685.