Improvement to 'hook' command, plus 'impl' command

This patch accomplishes a number of things:

1. Renames 'hook' to 'fn-hook', aliased to 'hook'.
2. Adds a new 'fn-impl' command, aliased to 'impl'. This function works like 'hook', but takes the full function name, skipping the multiple-choice selection that 'hook' has. Note that any function can be viewed in this way, not just hooks.
3. Shows the doxygen comment before the implementation of the function, if any.
4. Implements the function display code in php rather than sed

Naming conventions are somewhat debatable, I suppose. I was also planning 'fn-module', to show all of the hooks defined in a named module. That's not implemented yet, but it would make a suite of "fn" commands. I'm open for other suggestions.