tcp: refactor tcp_autorcvbuf() and associated sysctls

[glebius]

Summary

Run the entire function under the socket buffer lock. Make it responsible
to set the new limit or clear SB_AUTOSIZE in case of a failure. Use
unsigned type for newsize, that matches both the sb_hiwat type and min()
argument types. Make byte counter in the tcpcb unsigned, too. Make the
sizing tunables unsigned, and make the switching tunable a boolean. Don't
execute Dtrace probe in case of a failure. Simplify the Dtrace probe to
report only relevant information, the previous probe looked more like a
leftover from a debugging session.

[glebius]

Note that this conflicts with bugfix from @rrs that we are discussing with @tuexen in email and this revision will be updated.