Update dropt to v2.0.1

Primary motivation is updating the email address in the source code.
Also adjust some wording in README.md.

Add a new field to dropt_option to store additional callback data

Add a new field to dropt_option to store additional callback data.
This can be used to allow option handlers to store predefined values
to option variables (i.e., to offer functionality equivalent to
store_const from Python's argparse module).

Unfortunately, this warranted renaming the existing handler_data
member, so this might require adjusting any existing code that
initialized dropt_option members by name.  Since I'm already breaking
API compatibility for 2.0, I might as well do this now.

Since C does not guarantee that integers may be converted to pointers
and back, for maximum flexibility, the new member should be a
uintptr_t instead of a void*.  However, for compatibility with C89,
we can't use uintptr_t directly and must fall back to size_t.  Alas.

Change the dropt directory structure

Move header files into a separate "include" directory and most of the source
files into a "src" directory. (I made an exception for the example files which
currently remain in the main dropt directory.)

Also, I had forgotten to add most of the new files from r469 to the packaging
script.