irc/weechat: Update to 1.6

• Update to 1.6
• Take maintainership
• OPT_CMAKE_ON= -DENABLE_OPT=yes to OPT_CMAKE_BOOL= OPT
• Add LICENSE_FILE
• Drop BSD_LUA_* extra CMAKE_ARGS
• Add asciidoctor depend for DOCS and MANPAGES
• Fix build with Python 3.5 / Add warning
• Transform to post-patch-ICON-off target

PR: 205981
Reported by: Vladimir Krstulja <vlad-fbsd@acheronmedia.com>