logging: added debug_ipv4 and ipv4 trace flag
sysctl: prefer device config flags from newlink
sysctl: do not fail on read-only proc (bsc#904432)

Do not write sysctl settings which are already set and
do not fail on read-only /proc/sys/net, e.g. under LXC.
Merge pull request #470 from mtomaschewski/sysctls

sysctl: do not trigger ifup failure on read-only proc (bsc#904432)