NSCA-ng 1.6

• Work around TLSv1.3 PSK bug in (at least) OpenSSL 1.1.1b.
• Fix static linking against recent OpenSSL versions.
• Update our copy of libev from version 4.22 to version 4.25. The new release provides a few platform compatibilty fixes and small enhancements.
• Apply a few minor improvements.