networkd: make netdev_load_one "public" and add a fuzzer for it

src/network/netdev/netdev.c

@@ -594,7 +594,7 @@ int netdev_join(NetDev *netdev, Link *link, sd_netlink_message_handler_t callbac
         return 0;
 }
 
-static int netdev_load_one(Manager *manager, const char *filename) {
+int netdev_load_one(Manager *manager, const char *filename) {
         _cleanup_(netdev_unrefp) NetDev *netdev_raw = NULL, *netdev = NULL;
         _cleanup_fclose_ FILE *file = NULL;
         const char *dropin_dirname;

src/network/netdev/netdev.h

@@ -146,6 +146,7 @@ extern const NetDevVTable * const netdev_vtable[_NETDEV_KIND_MAX];
 #define NETDEV(n) (&(n)->meta)
 
 int netdev_load(Manager *manager);
+int netdev_load_one(Manager *manager, const char *filename);
 void netdev_drop(NetDev *netdev);
 
 NetDev *netdev_unref(NetDev *netdev);