.. and make travis run it.
I renamed check-license-headers.sh to check-style.sh and expanded it so
that it now also checks for the presence of "#pragma once" in .h files.
It also checks the presence of a (single) blank line above and below the
"#pragma once" line.
I also added "#pragma once" to all the files that need it: even the ones
we are not check.
I also added/removed blank lines in order to make the script not fail.
I also ran clang-format on the files I modified.
There are some headers in libc that require us to have definitions,
such as `FILE` available to us (such as in `pwd.h`). It is bad
practice to include the entirety of `stdio.h`, so it makes more
sense to put `FILE` into it's own header.