Enable tracking of pthreads condition variables

Enable tracking of pthreads condition variables

Some POSIX threads implementations (e.g. FreeBSD's libthr) allocate
memory on the heap when pthread_cond_init() is called.  Every call to
that function must be accompanied by a corresponding call to
pthread_cond_destroy() or else the memory allocated for the condition
variable will leak.

jemalloc can be used for detecting memory allocations which are not
released by a process when it exits.  Unfortunately, since jemalloc is
also the system allocator on FreeBSD and a special (profiling-enabled)
build of jemalloc is required for memory leak detection, this method
cannot be used for detecting leaked memory allocated by libthr on a
stock FreeBSD installation.

However, libthr's behavior can be emulated on any platform by
implementing alternative versions of libisc functions for creating and
destroying condition variables that allocate memory using malloc() and
release it using free().  This enables using jemalloc for detecting
missing pthread_cond_destroy() calls on any platform on which it works
reliably.

When the newly introduced ISC_TRACK_PTHREADS_OBJECTS preprocessor macro
is set, allocate isc_condition_t structures on the heap in
isc_condition_init() and free them in isc_condition_destroy().  Reuse
existing condition variable macros (after renaming them appropriately)
for other operations.