Improve checking for pthread_mutex_cond operations: implement a check

Improve checking for pthread_mutex_cond operations: implement a check
for consistent binding between the CV and the mutex, as specified by
POSIX.  Add commented out code for some other checks that could be
done but aren't, as they'd give false positives.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@10651