check-local target to enforce doxygen-correctness

We should have a check-local target that makes sure we haven't regressed anything in Doxygen. I'm going to compile a list of what that means here, and then make the target later in November.

Current properties are:

• Every C and H file (outside of ext, trunnel, and test) should have a @file or \file declaration.
• @file or \file declarations should match the actual name of the file.
• There should be no doxygen warnings (except for missing documentation).