irc/znc: Improve and simplify port

* Always build with ICU support
* Remove DEBUG menu option, use ports framework instead
* Always enable IPv6, kernel support is always enabled
* Always enable Python module as Python is already a hard dependency
* Do some small layout enhancements to port Makefile

This also aligns it more with other distros that packages znc

PR:		281346
Approved by:	portmgr (maintainer timeout, 2+ weeks)