tcp.4: Add missing sysctls, other fixes

Add some of the missing sysctls to tcp.4, using references to other
man pages where they exist. Added sysctls include recvbuf and sendbuf
controls for automatic buffer sizing. Updated recvspace and sendspace.

Add sysctl.8 to "see also" and intro to variable section. Rename
"MIB Variables" section to "MIB (sysctl) Variables", as most people
will associate with sysctl.

Reviewed by: manpages(pauamma), tuexen
Differential Revision: https://reviews.freebsd.org/D36004