net-irc/znc: bump to 1.6.3, EAPI=6

net-irc/znc: bump to 1.6.3, EAPI=6

- use the proper LICENSE
- remove the ZNC_DATADIR variable
- the libressl patch is no longer required → drop patch
- swig is not needed to build bindings from a release, removed from DEPEND
- configure uses python3 by default → don't specify it any more
- move informational postinst messages to a README.gentoo file
- fix the systemd unit by setting a valid HOME for the znc user
- delete the custom src_unpack function, provide the full path to gtest

Gentoo-Bug: https://bugs.gentoo.org/567344
Gentoo-Bug: https://bugs.gentoo.org/571366
Gentoo-Bug: https://bugs.gentoo.org/521916

Package-Manager: portage-2.2.28
Closes: https://github.com/gentoo/gentoo/pull/1086