Merging a number of PRs into one

See: http://developer.android.com/ndk/downloads/revision_history.html

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.

Signed-off-by: David Henningsson <diwic@ubuntu.com>

SYS_entropy is internal and may more easily change than getentropy(2).

- under openbsd/bitrig it is 256
- change type to `size_t` where system have `getnameinfo()` with
  `hostlen` as `size_t`

under openbsd it is a `caddr_t` which expand to `char *`.
as the size of the type change, adjust the padding too.

- openbsd
- bitrig
- netbsd

arguments `hostlen' and `servlen' are `socklen_t' or `size_t' depending
the platform

- apple: I don't known, so I don't change the original definition (socklen_t)
- netbsd: socklen_t
- freebsd: size_t
- dragonfly: size_t
- openbsd: size_t
- bitrig: size_t

- sysctl: *mut -> *const
- mprotect: *const -> *mut
- remove sysctlbyname() that don't exists under openbsd

- same prototype on bitrig and openbsd
- for netbsd, I did found it at all (?)

- IPV6_{ADD,DROP}_MEMBERSHIP
- HW_AVAILCPU

- under openbsd and bitrig: dli_saddr member is *mut ::c_void
- under netbsd: it is *const ::c_void

Add emscripten support

openbsd: map `si_addr` inside `struct siginfo_t`

formally, under OpenBSD, `si_addr` is part of a C union. As we don't use
others members of the union inside `struct siginfo_t`, it should be safe
to directly map the field.

Update the android CI image

It's got a new toolchain which should hopefully support some more modern
definitions.

Add getentropy(2)'s syscall number as a const