dir.c: allow 21.7303 to return NC, ax=0 to mean supported (EDR-DOS)

EDR-DOS returns NC, AX = 0000h from 21.7303. This was incorrectly
detected as "not supported" here. CF unchanged and AL = 00h is
the typical error return, albeit CF unchanged AX unchanged has
also been observed (on dosemu2 + lDOS + SRDISK).

Reference on the correct "unsupported" return:
https://sourceforge.net/p/freedos/mailman/message/59128382/

Incorrect interrupt list entry:
https://fd.lod.bz/rbil/interrup/dos_kernel/217303.html

Author: ecm-pushbx

cmd/dir.c

@@ -772,8 +772,10 @@ static int dir_print_free(unsigned long dirs)
 
   /* Note: RBIL carry clear and al==0 also means unimplemented 
      alternately carry set and ax==undefined (usually unchanged) for unimplemented
-  */  
-  if(!( r.r_flags & 1 ) && ( r.r_ax & 0xFF) ) {
+     ecm: RBIL is wrong, CF unchanged al=0 is the typical error return.
+     EDR-DOS returns NC ax=0 so checking for al!=0 here was wrong.
+  */
+  if(!( r.r_flags & 1 ) && ( r.r_ax != 0x7300 ) ) {
 	dprintf(("[DIR: Using FAT32 info]\n"));
 	clustersize = FAT32_Free_Space.sectors_per_cluster
 	 * FAT32_Free_Space.bytes_per_sector;