lwip server needs to include struct udp_io_hdr but must not include
struct udp-hdr as it conflicts with its internal type. So it is split
into to files now.
This patch moves further includes (the network part and lib.h) in common/.
It is the last part to get the netbsd libc to compile under minix. Further moves will be needed as we get the netbsd libc to compile minix itself.
Also, this patch add #ifndef's to termios.h, as it create problems with netbsd's namespace.h.