This patch adds support for the new option ALLOW_SYMLINK which can be used to specify which files are allowed to have a file type mismatch if the mismatch is caused by a symbolic link to the correct type. If the installation target is changed via DESTDIR, the symlink is not allowed to escape from it.
ALLOW_SYMLINK is hardcoded to always allow /boot to mismatch, since a bsdinstall configurations exist that create /boot as symlink to an unencrypted boot pool. In this case ignoring the mismatch during mergemaster causes mergemaster to also ignore - and thus not update - any files below /boot.
It is hardcoded to avoid introducing a mergemaster configuration file newly into the base install.