Add a `--license` command line argument

Add a new argument that allows users to pass in the path to a license
file. Doing so should get picked up by the Objective-C code which will
copy and validate the license.

In order for the Objective-C code to pick up the path string, we add a
`license` field to the `Config.Args` struct.

The new `license` field is a `*mut c_char`, which means we can't derive
Default any more and need to implement a custom `Default` impl for
`Args`.

Additionally, 'getopts' gives us a Rust string (of course), which needs
to be converted into a raw `CString` before we can stick it in the
`license` field in `Args`. I previously wrote `dkeprintln` to be able to
print an error here at the conversion point between `CString` and raw C
string.

Since we used `CString#into_raw()`, we need to call `from_raw()` on the
pointer to free it, otherwise we end up with a memory leak. This
necessitated adding some additional code to `config_free()` to free the
`license` field if non-null.

In order to access `config.args.license` from within `config_free()`, I
also needed to make the `args` and `license` struct fields public, since
the free function is in a different module. Instead of making only those
two fields public, I decided to make them all public, as all the fields
are technically accessible from C code, and are needed by that C code.

Finally, move the module declaration of `prefix_println` to the top of
the module list instead of listing it alphabetically, because otherwise
any modules declared before it (like `config`) don't get access to its
macro. Also add a `#[macro_use]` attribute to the module declaration
because we do want to export its macro.
(https://stackoverflow.com/questions/26731243/how-do-i-use-a-macro-across-module-files#comment78329437_31749071)

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