Buffer recv() calls.

Also puts in the place the more general helpers we want for adding more socketcall wrappers, but there's not a huge reason too atm:

Syscall histogram
<pre>
syscall	count
-------	-----
    240	 13408
    168	 12350
      5	  4020
    146	  3678
    265	  2688
    102	  1850  (socketcall)
    140	  1442
...
</pre>
buffer-flushers
<pre>
why	count
-------	-----
    168	  4572
    240	  2549
  -1019	  1614
    265	   776
    140	   498
    295	   270
    195	   207
    102	   137
    192	   117
...
</pre>

`connect` and `getsockname` make up well over 50% of the remaining socketcalls.