This solves a number of problems:

1. It gives a link to the source code at the top of the buffer, for
users who want to jump to that quickly. See discussion in #67.

2. It favours giving the filename of the source code in the beginning
of the buffer, as discussed in #69. However, we still show the full
path in the source code section. This avoids confusion when users have
upgraded built-in libraries (e.g. org-mode in ~/.emacs.d/elpa).

3. It ensures that buttons to source code files always jump to the
definition location wherever possible.

4. It highlights functions that are interactive or autoloaded, and
links to the relevant parts of the manual to clarify what that means.

edebug info may be a marker or a list, ensure we handle both.

If find-function-C-source-directory isn't set, or if it's set to a
directory that contains source to a different Emacs version, just
return nil from helpful--definition.

This fixes a number of issues spotted in the docstring of vhdl-mode.

Fixes #88

When we are opening a buffer for source code, we're essentially in
batch mode.

Previously the variable docstring had already been passed through
substitute-command-keys and it was too late for our button conversion.

The refactoring in 53dd618 checked that 'created' was t before we
actually set 'created'.