MINOR: args: add type-specific flags for each arg in a list

These flags are meant to be used by arg checkers to pass out-of-band
information related to some args. A typical use is to indicate how a
regex is expected to be compiled/matched based on other arguments.
These flags are initialized to zero by default and it is up to the args
checkers to set them if needed.

include/types/arg.h

@@ -92,6 +92,7 @@ union arg_data {
 struct arg {
 	unsigned char type;       /* argument type, ARGT_* */
 	unsigned char unresolved; /* argument contains a string in <str> that must be resolved and freed */
+	unsigned char type_flags; /* type-specific extra flags (eg: case sensitivity for regex), ARGF_* */
 	union arg_data data;      /* argument data */
 };