Add fastboot_test.cpp and test --os-version/--os-patch-level parsing.

Also switch all remaining headers over to #pragma once, and actually
use FB_COMMAND_SZ.

Bug: http://b/77340848
Test: ran tests
Change-Id: I27107d054c206e66c39208099e36a55df604e08f
diff --git a/fastboot/socket.h b/fastboot/socket.h
index 7eaa0ab..e791f2c 100644
--- a/fastboot/socket.h
+++ b/fastboot/socket.h
@@ -30,8 +30,7 @@
 // engine should not be using this interface directly, but instead should use a higher-level
 // interface that enforces the fastboot protocol.
 
-#ifndef SOCKET_H_
-#define SOCKET_H_
+#pragma once
 
 #include <functional>
 #include <memory>
@@ -125,5 +124,3 @@
 
     DISALLOW_COPY_AND_ASSIGN(Socket);
 };
-
-#endif  // SOCKET_H_