sysctl-util: minimize side-effects when running `systemd-sysctl`

sysctl-util: minimize side-effects when running `systemd-sysctl`

Currently `systemd-sysctl` binary is used in `systemd-sysctl.service`
which is mostly configured as `oneshot`. There are situations where one
would like to use systemd to maintain Sysctl configurations on a host,
using a configuration managers such as Chef or Puppet, by apply
configurations every X duration.
The problem with using `systemd-sysctl` is that it writes all the Sysctl
settings, even if the values for those settings have not changed. From
experience, we have observed that some Sysctl settings cause actions in
the kernel upon writing(like dropping caches) which in turn cause
undesired side effects.
This patch tries to minimize such side effects by comparing values
before writing.