Check header guards

This adds a header-guard converter and checker. It converts existing header guard to either

#ifndef PETSC_SRC_FULL_PATH_TO_FILE_AND_EXT
#define PETSC_SRC_FULL_PATH_TO_FILE_AND_EXT

or to

#pragma once

It also is able to check header guards once it has converted them, so can be added to e.g. checkbadSource to enforce the style going forward.