MFC r218912,218945,220237:

MFC r218912,218945,220237:

- Add new man pages for the modular congestion control, Khelp and Hhook
  frameworks (cc.4, cc.9, khelp.9 and hhook.9).

- Add new man pages for each available congestion control algorithm (cc_chd.4,
  cc_cubic.4, cc_hd.4, cc_htcp.4, cc_newreno.4 and cc_vegas.4).

- Add a new man page for the Enhanced Round Trip Time (ERTT) Khelp module
  (h_ertt.4).

- Update the TCP (tcp.4) man page to mention the TCP_CONGESTION socket option,
  cross reference to cc.4 and remove references to the retired
  "net.inet.tcp.newreno" sysctl MIB variable.

In collaboration with:	David Hayes <dahayes at swin edu au> and
  				Grenville Armitage <garmitage at swin edu au>
Sponsored by:	FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/8@222419 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f