Recent feature detection changes have gotten the -C (TCP congestion
algorithm selection) option to work on FreeBSD for free, starting with
FreeBSD 9.  Update various documentation places to note this.  One
specifies the congestion algorithm in the same was on Linux, although
the names of the algorithms are (at least in the general case) different.
"sysctl net.inet.tcp.cc" on FreeBSD provides a list of available
algorithms, which are implemented as loadable kernel modules.

Rename the --linux-congestion long option to --congestion (retaining
the old option as a deprecated synonym).