player: Drop long-disabled code to resolve IP addresses

Disabled since commit 32fac04 (v4.2.13) because it could at the time
use more stack space than we provided.  Additional issues: code still
uses obsolete gethostbyaddr() rather than getnameinfo(), and we
provide only 512 bytes for host names instead of the customary
NI_MAXHOST (1025) bytes.

All three issues would be easy enough to fix.  What's not so easy is
to avoid blocking on the synchronous DNS lookup.  Without that,
connecting repeatedly from a range of addresses with slow reverse
lookup could conceivably be employed as a denial of service attack.

We've been living without reverse lookup for close to ten years.  Bury
the corpse, and move on.

Bonus: sizeof(struct natstr) is cut in half.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

include/player.h

@@ -59,7 +59,6 @@ struct player {
     struct emp_qelem queue;
     empth_t *proc;
     char hostaddr[46];
-    char hostname[512];		/* may be empty */
     char client[128];		/* may be empty */
     char userid[32];		/* may be empty */
     int authenticated;