Signed-off-by: Arvid E. Picciani <aep@exys.org>

Include ioctl commands for becoming controlling tty and setting window
size.  This enables https://github.com/jwilm/alacritty to be built and
run on OpenBSD 6.3.  NetBSD and kin share the same command constants.

Add network interface flag constants for all platforms.

Rebase of #610 and also move emscripten up much higher in the hierarchy to
ensure that it doesn't have too much of a ripple effect on other platforms.

This involved moving down a good number of definitions, but hopefully was done
with care to not break anything!

It's not supported on those platforms

https://github.com/rust-lang/libc/pull/501

See http://www.catb.org/esr/time-programming/#_clock_gettime_2_clock_settime_2_clock_getres_2

factorize the definition with NetBSD

The new directory structure better reflects the historical relationship
of the BSDs, and includes 400 fewer duplicative lines of code.

Following types and function have been added:
* pthread_cond_init
* pthread_condattr_t
* pthread_condattr_init
* pthread_condattr_destroy
* pthread_condattr_setclock [0]
* pthread_condattr_getclock [0][1]

[0] Not available on OSX.
[1] May be unavailable on NetBSD, it is a very recent addition.
    Left out from openbsdlike hierarchy for now.