Merge branch 't/buffer_tree_improvements'

Was cooking for about a month.

	bcea04 btr: Simplify btr_node_status().
	072391 buffer_tree: Improve btr_splice_out_node().

diff --combined NEWS
index d1155022a36e1c1ccf623c5eea59fd514463ed2f,7cd6838e008319f07a83e58aab3aa3ed98090b94..fc6ccffb03a9ffc1903d66208bb097fb2e54fc64
--- 1/NEWS
--- 2/NEWS
+++ b/NEWS
@@@ -1,12 -1,6 +1,13 @@@
  NEWS
  ====
  
 +----------------------------------------------
 +0.5.1 (to be announced) "temporary implication"
 +----------------------------------------------
 +
 +      - audiod improvements and fixes.
++      - buffer tree robustness improvements.
 +
  ----------------------------------------
  0.5.0 (2013-08-23) "invertible validity"
  ----------------------------------------