network/netdev: use hashmap_remove_value() on detaching NetDev from manager

Then, it is not necessary to free NetDev.ifname when a conflicting
.netdev file is already loaded.

This also split out netdev_detach_name() and netdev_detach_impl().

No functional change, just refactoring.

src/network/netdev/netdev.h

@@ -195,6 +195,9 @@ extern const NetDevVTable * const netdev_vtable[_NETDEV_KIND_MAX];
 /* For casting the various netdev kinds into a netdev */
 #define NETDEV(n) (&(n)->meta)
 
+NetDev* netdev_detach_name(NetDev *netdev, const char *name);
+void netdev_detach(NetDev *netdev);
+
 int netdev_load(Manager *manager, bool reload);
 int netdev_load_one(Manager *manager, const char *filename);
 void netdev_drop(NetDev *netdev);