Log Message:
gsed related clean up.
* Stop to treat NetBSD's sed as GNU sed, not full compatible.
  * Then, no need to reset TOOLS_PLATFORM.gsed for NetBSD if USE_TOOLS+=gsed and
    real GNU sed is required.
  * In addition, convert simple USE_TOOLS+=gsed to conditionally, without NetBSD.
* convert {BUILD_,}DEPENDS+=gsed to USE_TOOLS, all tools from gsed are real gsed.