LXC: normalize package installation & user creation.

utils/lib.sh:
- get DIST_ID & DIST_VERSION from /etc/os-release
- pkg_[install|remove|...] supports ubuntu, debian, archlinux & fedora

utils/lxc.sh
- Workaround for the "setrlimit(RLIMIT_CORE): Operation not permitted" error::
    'Set disable_coredump false' >> /etc/sudo.conf

utils/[searx.sh|filtron.sh|morty.sh]
- switched user creation from 'adduser' perl script to 'useradd' built-in
  command

utils/searx.sh
- install packages for ubuntu, debian, archlinux & fedora

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>