add basic support for retry; protect internal bind behind mutexes; rework search, review candidate selection; minor cleanup.  Passes concurrency test036

-regenerated autoconf and automake files
-additional example code

   context checking and multiple match resolution via an internal search
b) move the candidate listing in a persistent per-thread buffer
c) fix bind procedure
d) minor cleanup

(a) and (b) should address ITS#2935; (b) and (c) should address ITS#3171

A test is also added