be consistent with how we format headers

[ppannuto]

We were close to a consensus on most of this, but now everything
follows the same format:

`#pragma once` directive
[newline]
System headers in <>'s
[newline]
Tock headers in ""'s
[newline]
C++ guard
[newline]
#define's
[newline]
function prototypes
[newline]
C++ guard

The only real changes / decisions:

• #pragma once instead of include guards. We were about 50/50.
  I think the pragma looks cleaner and is easier, so I went that way.
• C++ open guard above defines. We were about 75/25 this way already.
  I think this is easier to parse as it groups the things this header
  is defining together instead of putting this random build artifact
  in the middle
• Add missing C++ guards. Only a few files had forgotten them. I don't
  think any files were #define only, but even if they were, I'd be
  inclined to still include the guards to be consistent across all
  headers and more importantly to reduce the chance that it's forgetten
  if a method is added.

[alevy]

Agree with all the points...