net-mgmt/semaphore: update 2.4.1 to 2.5.1

• Makefile
  
  adopt the port; pet portlint; add LICENSE; use USES=go (introducing GO_ENV and GO_BUILDFLAGS); push vendor tree to 'public/vendor' (upstream change); offer MYSQL support (defined by upstream).
• files/*
  
  patch-util_config.go -> patch failing 'printVersion' (undef var).

Approved by: rene (mentor)
Differential Revision: https://reviews.freebsd.org/D19045