net/libnfs: Update to 5.0.2

ChangeLog: https://github.com/sahlberg/libnfs/releases/tag/libnfs-5.0.2

Mostly minor fixes for multithreading and compilation on *BSD.

PR: 265822
Reported by: vvd@unislabs.com
Approved by: snoe925@gmail.com (maintainer, timeout > 2 weeks)