Introduce job flags

Introduce job flags, beginning with a "full coinbase" flag (enabled by default) to indicate the entire generation transaction is included in the AVA2_P_COINBASE data; if the software unsets this flag, the data is always interpreted as a SHA256 midstate followed by datatail
    
A negative flag has been chosen to remain compatible with the prior AVA2_P_LONGCOINBASE protocol change

Over-long generation transactions remain interpreted as midstate+datatail even if the "full coinbase" flag is set

NOTE: Untested.