Add SO_REUSEPORT for Android
Add SO_REUSEPORT for Android

The `net2` crate currently fails to cross-compile to Android with
``error: unresolved name `libc::SO_REUSEPORT` ``

According to http://stackoverflow.com/a/14388707/1162888,
Android is essentially the same as Linux for the purpose of
socket features support, and the Linux kernel has supported
`SO_REUSEPORT` since version 3.9.

According to http://android.stackexchange.com/a/51656,
Android has had more recent kernels than that since version 4.4 Kit Kat.

anatol/libc,mneumann/libc,cactorium/libc,philippkeller/libc,anatol/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang/libc,cybergeek94/libc,philippkeller/libc,superriva/libc,philippkeller/libc,superriva/libc,rust-lang-nursery/libc,zethra/libc,cybergeek94/libc,anatol/libc,mneumann/libc,cybergeek94/libc,mneumann/libc,rust-lang/libc,cactorium/libc,rust-lang/libc,cactorium/libc,superriva/libc,zethra/libc,zethra/libc