net-im/toxic: drop unused dependencies, unbreak PYTHON option

• Remove security/libsodium and audio/opus dependencies;

the port does not reference nor links against them.

• Allow to use ncurses from base (if possible).
• Patch python3 detection to use proper pkgconfig/python3-config and

bin/python3 paths.

• Move USES upper (portlint).
• Bump PORTREVISION.

PR: 239215
Approved by: portmgr (blanket(s): dependencies, build fix(es))
MFH: 2019Q3