ifupdown: support "source-directory" stanza

ifupdown: support "source-directory" stanza

Support the "source-directory" stanza from ifupdown[1]. source-directory
will include all files in the named directory. Similar to the Busybox
version of the "source" stanza, this version of source-directory does
not currently support shell wildcards.

We only check that the stanza starts with "source-dir" as ifupdown does[2].

[1] https://manpages.debian.org/stretch/ifupdown/interfaces.5.en.html#INCLUDING_OTHER_FILES
[2] https://salsa.debian.org/debian/ifupdown/blob/0.8.33/config.c#L498

function                                             old     new   delta
read_interfaces                                     1150    1241     +91

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>