source directive support is broken

[mstone232]

If /etc/network/interfaces contains something like "source interfaces.d/*" the files in interfaces.d are read, but when interfaces is rewritten the directive is removed and nothing in interfaces.d is read in future. (Either by debinterface or ifupdown -- the system is potentially badly broken.) Additionally if the newer "source-directory interfaces.d" syntax is used, the directive is completely ignored by debinterface, as well as being removed on write.

There are traces of e.g., _write_sourced_paths, in the code, but that function is never called; I'm not entirely sure what the expected state/use is so I'm hesitant to propose a patch.