xz: Change the way coder_run() and list_run() are called in main().

xz: Change the way coder_run() and list_run() are called in main().

Previously, a function pointer was used to determine if coder_run() or
list_run() should be called in the main entry processing loop. This was
replaced by an extra function call to process_entry().

coder_run() and list_run() were changed to accept a file_pair * argument
instead of a filename. The common repeated code was moved to
process_entry() instead.