Various kinds of input stashing were replaced with a single buflist before
v3.0... this patch replaces the partial send arrangements with its own buflist
in the same way.
Buflists as the name says are growable lists of allocations in a linked-list
that take care of book-keeping what's added and removed (even if what is
removed is less than the current buffer on the list).
The immediate result is that we no longer have to freak out if we had a partial
buffered and new output is coming... we can just pile it on the end of the
buflist and keep draining the front of it.
Likewise we no longer need to be rabid about reporting multiple attempts to
send stuff without going back to the event loop, although not doing that
will introduce inefficiencies we don't have to term it "illegal" any more.
Since buflists have proven reliable on the input side and the logic for dealing
with truncated "non-network events" was already there this internal-only change
should be relatively self-contained.
