IPv4: Support overriding the default TTL (64)

Made getsockopt() and setsockopt() virtual so we can handle them in the
various Socket subclasses. The subclasses map kinda nicely to "levels".

This will allow us to implement things like "traceroute", although..
I spent some time trying to do that, but then hit a wall when it turned
out that the user-mode networking in QEMU doesn't preserve TTL in the
ICMP packets passing through.

Kernel/UnixTypes.h

@@ -337,10 +337,13 @@ struct pollfd {
 #define SO_KEEPALIVE 3
 #define SO_ERROR 4
 
+#define IPPROTO_IP 0
 #define IPPROTO_ICMP 1
 #define IPPROTO_TCP 6
 #define IPPROTO_UDP 17
 
+#define IP_TTL 2
+
 struct sockaddr {
     u16 sa_family;
     char sa_data[14];