Add a flag to disable FIFO probing. The code seems to have a chance of

Add a flag to disable FIFO probing.  The code seems to have a chance of
misdetecting FIFO capabilities, at least on my girlfriend's Thinkpad 755,
the driver doesn't work using the FIFO.

While i was at it, i (partially) fixed option FCC_YE since it would no
longer have compiled at all under -current.  I've also made an attempt
to document the device driver flags value (ab-)used internally by this
option.

RELENG_3 candidate, but with a slightly different patch there (will go
to jkh in email).