Revert r274494, r274712, r275955 and provide extra comments explaining
why there could appear a zero-sized mbufs in socket buffers.

A proper fix would be to divorce record socket buffers and stream
socket buffers, and divorce pru_send that accepts normal data from
pru_send that accepts control data.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275968 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f