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/nat.h

@@ -31,7 +31,7 @@
  *     Ken Stevens, 1995
  *     Steve McClure, 1998-2000
  *     Ron Koenderink, 2005-2008
- *     Markus Armbruster, 2005-2013
+ *     Markus Armbruster, 2005-2014
  */
 
 #ifndef NAT_H
@@ -95,7 +95,6 @@ struct natstr {
     char nat_cnam[20];		/* country name */
     char nat_pnam[20];		/* representative */
     char nat_hostaddr[46];	/* host addr of last user */
-    char nat_hostname[512];	/* hostname of last user, may be empty */
     char nat_userid[32];	/* userid of last user, may be empty */
     coord nat_xcap, nat_ycap;	/* cap location in abs coords */
     coord nat_xorg, nat_yorg;	/* origin location in abs coords */