Previously, the oob data was just stored and sent to WriteMsgUDP but it
ignores the Src field when writing. Instead, now it is setting the Src
to the original Dst and handling IPv4 IPs over IPv6 correctly.
App Engine even though is Linux, does not allow referencing the syscall package. Thus I reorganised
the udp*.go files a little to make this work. As best I can the functionality is unchanged, and all
this continues to compiles on darwin, linux, windows, plan9 and appengine.
Namely:
* Moved all Linux specific code from udp.go into udp_linux.go (e.g setUDPSocketOptions)
* Deleted udp_plan9.go which was almost a copy of udp.go.
* Added build tags to stop appengine using the linux build, and instead using the udp_other.go.
