Provide new socket option TCP_CCALGOOPT, which stands for TCP congestion
control algorithm options. The argument is variable length and is opaque
to TCP, forwarded directly to the algorithm's ctl_output method.

Provide new includes directory netinet/cc, where algorithm specific
headers can be installed.

The new API doesn't yet have any in tree consumers.

The original code written by lstewart.
Reviewed by: rrs, emax
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D711