Wrap two of the top buffer-flushing syscalls.

After this, the top 10 are
<pre>
why	count
-------	-----
-1019	   815  (time slice)
 240	   409  (futex)
 295	   241  (un-wrappable openat)
 195	   184  (un-wrappable stat)
 168	   177  (poll)
 102	   172  (socketcall)
 140	   144  (_llseek)
 265	   104  (clock_gettime)
 192	    71  (mmap2)
   5	    55  (un-wrappable open)
</pre>

Starting to converge on the usual suspects seen in #242.

With this patch, FF started up under 24s for the first time.  So we're at about a 10-15% improvement in startup from adding a few more wrappers.