/e/n/i: move sourcing of /etc/network/interfaces.d/ to begin of file

netcardconfig doesn't properly handle this section and it disappears
when configuring network devices then.

Rewriting netcardconfig is on our agenda (see
grml/grml-network#11), until we get there
let's make sure the status quo works as good as possible.

Take over wording based on what ifupdown's postinst script uses.

etc/grml/fai/config/files/etc/network/interfaces/GRMLBASE

@@ -2,8 +2,12 @@
 # ${GRML_FAI_CONFIG}/config/scripts/GRMLBASE/35-network script, using
 # ${GRML_FAI_CONFIG}/config/files/etc/network/interfaces/GRMLBASE
 
+# interfaces(5) file used by ifup(8) and ifdown(8)
+
+# Include files from /etc/network/interfaces.d:
+source /etc/network/interfaces.d/*
+
 # The loopback interface
 auto lo
 iface lo inet loopback
 
-source /etc/network/interfaces.d/*