Support for virtual filenames in library

Commandline usage will set the virtual filename to be equal to the actual filename, but calling programs are free to set it to a different value.
Rename diskfile_t to spar_diskfile_t

Makes it more unique to this program. Calling applications might have already declared their own diskfile_t, which could lead to conflicts.
Makefile and configure changes

In preparation for (static) library support.
Move cli functions to separate file

We do not want to include main() and parse() in a static library as those are purely commandline functions.
Rename spar_get_packed function

It requires too much knowledge to be the default function to request packets. A simpler, more adequate function is needed.
Add some extra initial parameters
Clarify the need for including par2.h when using recvslice_get
Easy library interface for adding input files

Please make sure that all input files are added before opening the recovery file generator.`
Easy library interface to get packets

A simple, but more importantly, thread-safe method to request packets in order.
Interface method to get filenames and packet counts

This information was stored in the handle, which would require including par2.h to manipulate or access. Now this information is properly exposed and accessible by means of an interface method.