rping: create persistent server threads in DETACHED state

rping: create persistent server threads in DETACHED state

Since the persistent server threads aren't joined, they must be created in
the DETACHED state or resources will not be cleaned up when they exit.
This results in pthread_create() failures after thousands of rping
instances are run against a persistent server.

Also check the return from all calls to pthread_create() so we don't
ignore a thread creation failure.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>