Have pthread_cond_destroy() return EBUSY if the condvar has waiters.

Have pthread_cond_destroy() return EBUSY if the condvar has waiters.

This is not required of a compliant implementation, but it's easy to
check for and helps improve compatibility with other common
implementations.  Moreover, it's consistent with our
pthread_mutex_destroy().

PR:		234805
Reviewed by:	jhb, kib, ngie
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D19496