sendfile: factor out socket send buffer space sensing into a method

Move a block of code that works with the socket send buffer from the main
sendfile loop into a separate function. Make it a protocol method, so
that protocols may provide a different one.

While here, provide a long comment explaining why we modify sb_lowat and
why we can't just remove that hack.

No functional change intended.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D48918