Start work on making C64 formats encoding possible by changing DMDecodeOps to DMEncDecOps and adding fields and op enums for custom encode functions, renaming, etc. Split generic op sanity checking into a separate function in preparation for its use in generic encoding function.

#include "dmargs.h"
#define TH_EXTERNAL 1
#define optarg_t DMOptArg
#define th_args_process dmArgsProcess
#define th_args_help dmArgsPrintHelp

#define THERR dmError
#define th_calloc dmCalloc
#define th_malloc dmMalloc
#define th_free dmFree

#include "dmargs_int.c"