sysctl-util: rework sysctl_write() to wrap write_string_file()

sysctl-util: rework sysctl_write() to wrap write_string_file()

The sysctl_write_ip_property() call already uses write_string_file(), so
let's do so here, too, to make the codepaths more uniform.

While we are at it, let's also validate the passed path a bit, since we
shouldn't allow sysctls with /../ or such in the name. Hence simplify
the path first, and then check if it is normalized, and refuse if not.