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