devel/libzvbi: modernize

• Pass -DLIBICONV_PLUG [1]
• Make NLS optional [2]
• Convert to USES=localbase
• Don't try to detect Doxygen
• Strip binaries
• Drop unused USES=pkgconfig
• Define LICENSE

PR: 206966
Reported by: vvd@unislabs.com (via koobs) [1]
Approved by: oliver (maintainer) [1][2], portmgr blanket