Include netplug into beyond-net

[Andrew Bradford]

Since ifplugd in busybox doesn't compile cleanly with musl, let's use netplug (which is also quite small) for bringing the network up and down on Ethernet cable insert/removal. netplug should go in beyond-net.

Of note, netplug does not compile cleanly and is in need of some patching. For instance the netplug.h file uses time_t but doesn't #include <time.h> which throws an error and libc.c #includes <assert.h> but then redefines __assert_fail. These don't show errors native compiling with eglibc but with musl cross building they are errors that should be fixed.