basic/in-addr-util: add IN_ADDR_TO_STRING

Since we don't need the error value, and the buffer is allocated with a fixed
size, the whole logic provided by in_addr_to_string() becomes unnecessary, so
it's enough to wrap inet_ntop() directly.

inet_ntop() can only fail with ENOSPC. But we specify a buffer that is supposed
to be large enough, so this should never fail. A bunch of tests of this are added.
This allows all the wrappers like strna(), strnull(), strempty() to be dropped.

The guard of 'if (DEBUG_LOGGING)' can be dropped from around log_debug(),
because log_debug() implements the check outside of the function call. But
log_link_debug() does not, so it we need it to avoid unnecessary evaluation of
the formatting.

src/nspawn/nspawn-expose-ports.c

@@ -141,14 +141,9 @@ int expose_port_execute(sd_netlink *rtnl, FirewallContext **fw_ctx, ExposePort *
         if (in_addr_equal(af, exposed, &new_exposed))
                 return 0;
 
-        if (DEBUG_LOGGING) {
-                _cleanup_free_ char *pretty = NULL;
-                in_addr_to_string(af, &new_exposed, &pretty);
-                log_debug("New container IP is %s.", strna(pretty));
-        }
+        log_debug("New container IP is %s.", IN_ADDR_TO_STRING(af, &new_exposed));
 
         LIST_FOREACH(ports, p, l) {
-
                 r = fw_add_local_dnat(fw_ctx,
                                       true,
                                       af,