Meta: Add a script check the presence of "#pragma once" in header files

.. 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.

Libraries/LibCrypto/BigInt/UnsignedBigInteger.h

@@ -25,6 +25,7 @@
  */
 
 #pragma once
+
 #include <AK/ByteBuffer.h>
 #include <AK/LogStream.h>
 #include <AK/String.h>
@@ -45,7 +46,7 @@ public:
     {
     }
 
-    UnsignedBigInteger() {}
+    UnsignedBigInteger() { }
 
     static UnsignedBigInteger create_invalid();