src.conf.5: Use common phrasing in WITHOUT_OFED text

Reported by: jhb
Sponsored by: The FreeBSD Foundation

Drop 'Set to' from most src.conf(5) knobs

The description is clearly what effect the knob has when set, so the
additional text was unnecessary.

Reviewed by: jhb, se
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29583

WITH_OFED build option: fix

Userspace has OFED build enabled for quite some time, but kernel modules
were not. This is useless config because any userspace IB code requires
kernel support. So enable modules build by default.

Move WITH_OFED to WITHOUT_OFED since defaults are now enabled.

Reviewed by: emaste, hselasky, kevans
MFC after: 3 days
Sponsored by: NVidia Networking / Mellanox Technologies
Differential Revision: https://reviews.freebsd.org/D28460