CONFIG_PROTECT: handle non-existent files

This fixes the ConfigProtect class, etc-update, and dispatch-conf to
account for non-existent files (rather than directories) that are
listed directly in CONFIG_PROTECT. It has been valid to list files
directly in CONFIG_PROTECT since bug #14321. However, the support for
non-existent files added for bug #523684 did not include support for
non-existent files listed directly in CONFIG_PROTECT.

Fixes: 5f7b4865ecaf ("dblink.mergeme: implement bug #523684")
X-Gentoo-Bug: 523684
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=523684
Acked-by: Alexander Berntsen <bernalex@gentoo.org>