This reverts commit c124acba.

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!

* Now a complete signal list on Haiku minus reserved.
* Fixes build failure due to missing SIGUSR1

    - Copy 17 functions definitions from src/unix/mod.rs
      to src/unix/bsd/mod.rs src/unix/haiku/mod.rs
      src/unix/notbsd/linux/mod.rs and src/unix/solaris/mod.rs
    - Add some functions to android that was cfged out
    - Remove cf* and tc* functions implementations for android
      (they are available with api >= 12, which was release in 2011)

It turns out that *only* FreeBSD and NetBSD proper have waitid, and that
Solaris' additional W* constants are totally different from everyone
else's, which tips me over from 'find some way to shoehorn this into
the top-level unix/mod.rs' to 'put it in the submodules, live with the
code duplication'.

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