dblink.mergeme: implement bug #523684

If a CONFIG_PROTECTed file was installed but no longer exists in the
file system, then it may have been deleted or renamed by the admin.
Therefore, force the file to be merged with a ._cfg name, so that the
admin will be prompted for this update.

X-Gentoo-Bug: 523684
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=523684