math/nlopt: Update to 2.7.0

Minor release:

• New nlopt_set_param API for setting internal algorithm parameters
• Avoid library-symbol conflicts

Changelog: https://github.com/stevengj/nlopt/releases/tag/v2.7.0

Reported by: portscout
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D27276