Add cxgbetool(8) to the base system.

Move cxgbetool from tools/tools to usr.sbin. Compile and install it on
platforms where cxgbe(4) is built by default. Knobs (WITH_CXGBETOOL and
WITHOUT_CXGBETOOL) have been added so that the user can override the
default setting.

Reviewed by: ngie@, gnn@, bdrewery@
MFC after: 1 month
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D9854