- Get rid of proto.h and split out its declarations into new .h files matching the .c files in which the definitions will be found
- Use include guards in headers
- Reorganise a couple of functions to make them static

NB this is just the first step. Some of the additional headers can probably be obsoleted with further refactoring as they only contain one or two declarations - there's plenty of scope for better organisation of .c files (e.g. get_pass and other utility functions could be combined into a util.c).