enable KDB_TRACE in GENERICs

KDB_TRACE is not an alternative to DDB/etc, they are complementary.
So I do not see any reason to not enable KDB_TRACE by default.

X-MFC after:	never (change specific to head)
Improve r253721 by reporting detected lack of BIO_FLUSH support to GEOM.
That prevents more of such requests from coming and errors from logging.