rework parse so that it can process larger files.

dumpidx.c

@@ -2,11 +2,20 @@
 #include "mmap.h"
 #include "uint32.h"
 
-int main() {
+int main(int argc,char* argv[]) {
   int verbose=0;
   unsigned long filelen;
-  char* map=mmap_read("data",&filelen);
+  char* fn=argc<2?"data":argv[1];
+  char* map=mmap_read(fn,&filelen);
   uint32 magic,attribute_count,record_count,indices_offset,size_of_string_table;
+  if (!map) {
+    buffer_puts(buffer_2,"could not open `");
+    buffer_puts(buffer_2,fn);
+    buffer_puts(buffer_2,"´: ");
+    buffer_puterror(buffer_2);
+    buffer_putnlflush(buffer_2);
+    exit(1);
+  }
   buffer_puts(buffer_1,"magic: ");
   uint32_unpack(map,&magic);
   uint32_unpack(map+4,&attribute_count);