Provide file format I: Intel HEX with comments that ignores checksum errors

The new file type I is essentially Intel HEX that, on download, inserts
comments next to data records with the resolved effective address and an
ASCII dump of that same record. On upload the `I` format is permissive
with respect to check sum errors, eg, after manipulated an Intel HEX file
for debugging.

src/libavrdude.h

@@ -822,7 +822,8 @@ typedef enum {
   FMT_DEC,
   FMT_OCT,
   FMT_BIN,
-  FMT_ELF
+  FMT_ELF,
+  FMT_IHXC,
 } FILEFMT;
 
 struct fioparms {