macro: introduce TAKE_PTR() macro

This macro will read a pointer of any type, return it, and set the
pointer to NULL. This is useful as an explicit concept of passing
ownership of a memory area between pointers.

This takes inspiration from Rust:

https://doc.rust-lang.org/std/option/enum.Option.html#method.take

and was suggested by Alan Jenkins (@sourcejedi).

It drops ~160 lines of code from our codebase, which makes me like it.
Also, I think it clarifies passing of ownership, and thus helps
readability a bit (at least for the initiated who know the new macro)

src/network/netdev/netdev.c

@@ -488,8 +488,7 @@ int netdev_get_mac(const char *ifname, struct ether_addr **ret) {
         mac->ether_addr_octet[0] &= 0xfe;        /* clear multicast bit */
         mac->ether_addr_octet[0] |= 0x02;        /* set local assignment bit (IEEE802) */
 
-        *ret = mac;
-        mac = NULL;
+        *ret = TAKE_PTR(mac);
 
         return 0;
 }

src/network/netdev/wireguard.c

@@ -228,8 +228,7 @@ static int on_resolve_retry(sd_event_source *s, usec_t usec, void *userdata) {
 
         w->resolve_retry_event_source = sd_event_source_unref(w->resolve_retry_event_source);
 
-        w->unresolved_endpoints = w->failed_endpoints;
-        w->failed_endpoints = NULL;
+        w->unresolved_endpoints = TAKE_PTR(w->failed_endpoints);
 
         resolve_endpoints(netdev);