sysctl-util: make sysctl_write_ip_property() a wrapper around sysctl_write()

It does the same stuff, let's use the same codepaths as much as we can.

And while we are at it, let's generate good error codes in case we are
called with unsupported parameters/let's validate stuff more that might
originate from user input.

src/basic/socket-util.c

@@ -793,7 +793,7 @@ bool ifname_valid_full(const char *p, IfnameValidFlags flags) {
 
         /* Let's refuse "all" and "default" as interface name, to avoid collisions with the special sysctl
          * directories /proc/sys/net/{ipv4,ipv6}/conf/{all,default} */
-        if (STR_IN_SET(p, "all", "default"))
+        if (!FLAGS_SET(flags, IFNAME_VALID_SPECIAL) && STR_IN_SET(p, "all", "default"))
                 return false;
 
         for (const char *t = p; *t; t++) {