Use inc_flags instead of the inc_isipv6 alias which so far
had been the only flag with random usage patterns.
Switch inc_flags to be used as a real bit field by using
INC_ISIPV6 with bitops to check for the 'isipv6' condition.

While here fix a place or two where in case of v4 inc_flags
were not properly initialized before.[1]

Found by: rwatson during review [1]
Discussed with: rwatson
Reviewed by: rwatson
MFC after: 4 weeks