resolved: lower SERVFAIL cache timeout from 30s to 10s

Apparently 30s is a bit too long for some cases, see #5552. But not
caching SERVFAIL at all also breaks stuff, see explanation in
201d99584e.

Let's try to find some middle ground, by lowering the cache timeout to
10s. This should be ample for the problem
201d99584e attackes, but not as long as
half a miute, as #5552 complains.

Fixes: #5552

src/resolve/resolved-dns-cache.c

@@ -20,7 +20,7 @@
 
 /* How long to cache strange rcodes, i.e. rcodes != SUCCESS and != NXDOMAIN (specifically: that's only SERVFAIL for
  * now) */
-#define CACHE_TTL_STRANGE_RCODE_USEC (30 * USEC_PER_SEC)
+#define CACHE_TTL_STRANGE_RCODE_USEC (10 * USEC_PER_SEC)
 
 typedef enum DnsCacheItemType DnsCacheItemType;
 typedef struct DnsCacheItem DnsCacheItem;