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)
version 0.6.13