Add support for command-specific completion codes

Some commands may return command-specific completion codes.
Now they are all reported as 'Unknown'.
Add helper functions to support such command-specific codes.
Command handlers will need to define their own valstr arrays
with completion code descriptions and then use specific_val2str()
instead of generic val2str() to convert the completion code into
a string.

Also reduce code duplication in helper.c

Signed-off-by: Alexander Amelkin <alexander@amelkin.msk.ru>

include/ipmitool/helper.h

@@ -80,6 +80,10 @@ struct oemvalstr {
 	const char * str;
 };
 
+const char *
+specific_val2str(uint16_t val,
+                 const struct valstr *specific,
+                 const struct valstr *generic);
 const char * val2str(uint16_t val, const struct valstr * vs);
 const char * oemval2str(uint32_t oem,uint16_t val, const struct oemvalstr * vs);