Use the full MIDNSHP=X string whenever printing CIGAR strings

Define MM_CIGAR_STR to the full string of CIGAR operators (including
the 'B' operator as well) and use it throughout the C code.

It would be possible to use it from the Cython code too, but it's easier
to keep that as a Cython string literal to avoid adding extra runtime
code to handle locale conversion.

minimap.h

@@ -46,6 +46,8 @@
 
 #define MM_MAX_SEG       255
 
+#define MM_CIGAR_STR  "MIDNSHP=XB"
+
 #ifdef __cplusplus
 extern "C" {
 #endif