sysutils/liburcu: backport upstream patches for mutex leak

liburcu's use of pthread_mutex_init without a matching destroy causes
mutexs to be leaked. Backport some fixes from the upstream
stable-0.15/ branch to resolve the issue.

The memory leak this caused impacts bind, gdnsd, and other ports.

This is a subset of the changes from the commit to ports main (802d0c)
without updating the entire port or bumping the version.

PR: 296786, 293737
Co-authored by: Daniel Engberg <diizzy@FreeBSD.org>
Sponsored by: Klara, Inc.