include/netinet/in.h: Rename __pad member to sin_zero in struct sockaddr_in.

Libwebsockets initializes the 'sin_zero' member of sockaddr_in objects to zeros.
Apparently, judging from mailing list entries, there are platforms on which incliinclunot
doing this causes undefined behaviour.

On NuttX compiling respective code is broken, as the corresponding member
variable is called '__pad'.  Neither in the 'nuttx' nor in the 'apps'
respository did I find any reference to this identifier.  Thus, I believe its a
safe bet to just rename it.

UNIX Network Programming states in this regard:

"The POSIX specification requires only three members in the structure: sin_family,
sin_addr, and sin_port. It is acceptable for a POSIX-compliant implementation to
define additional structure members, and this is normal for an Internet socket
address structure. Almost all implementations add the sin_zero member so that all
socket address structures are at least 16 bytes in size."

include/netinet/in.h

@@ -222,7 +222,7 @@ struct sockaddr_in
   sa_family_t     sin_family;       /* Address family: AF_INET */
   in_port_t       sin_port;         /* Port in network byte order */
   struct in_addr  sin_addr;         /* Internet address */
-  uint8_t         __pad[8];
+  uint8_t         sin_zero[8];
 };
 
 /* Used with certain IPv4 socket options */