Add support for SUSE based distros in etc-update

Add support for reading/merging *.rpmnew files created in SUSE based distros.
Configuration from etc-update will be read:
- from portage config vars in Gentoo based distros
- from portage config vars in non Gentoo distros if portage is installed
- from /etc/sysconfig/etc-update in SUSE distros if portage is absent

Signed-off-by: Michal Hrušecký <miska@gentoo.org>
X-Gentoo-Bug: 456128
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=456128