This change adds missing features:

* %[ for pattern matching with a scanset.
* %m for allocation of strings.
* %p for pointers.
* Field width for integers.

The following features remain unimplemented, like with printf(3):

* Floating point support.
* Wide character support.
* %n$ positional parameters.

The code has been completely refactored to be much more maintainable.

The implemented features should now be standards compliant. A large number
of edge cases have been fixed.

The vscanf_callback(3) function has been renamed to vcbscanf(3) and a new
cbscanf(3) function has been added.