enableBBR silently fails if the kernel module was not properly loaded

[mattmathis]

Rather than making this fatal, consider 1) making sure that the actual congestion control is plumbed through to a.CongestionControl; 2) monitor CC statistics and 3) implementing #313 at the same time.

1. should probably be implemented with a syscall if at all possible. Beware that the BBRinfo selector is overloaded, so non BBR TCP might report nonsense (rather than a error).

Deferring 2) is no worse than our present state.