ifcfg/write-ifcfg.sh: turn on IPV6INIT, if any inet6 address is found

If "ip -6 addr" finds any inet6 address, assume IPV6INIT=yes for the
ifcfg file.
lvm:module-setup.sh: check for existance of 69-dm-lvm-metad.rules

Before modifying 69-dm-lvm-metad.rules, we should check for the
existance first. Otherwise this results in error messages on
distributions (debian), which do not ship these rules.
Break at switch_root only for bare rd.break

Previously, any rd.break=breakpoint would cause a break at the
given breakpoint and also at switch_root.