NAT: icmpwin_ping - don't decrement TTL once again, this is done in ip_input(), where time exceeded is also handled. Note, that this is not correct, since time exceeded should not be generated for packets with TTL 1 that are for "us", i.e. one of the CTL_ALIAS addresses. But for now just do as the Romans do.

scm automatic cleanups.

NAT: new Windows ping proxy that is not limited to just one outstanding ping of 360 bytes max. Mostly adapted from NAT Network pxping_win.c.

NAT: oops, g/c debug log in previous

NAT: allocate extra space for ICMP API internal use if reqest size is too small

NAT: fix in_cksum_skip() calls - "len" parameter must include the "skip"ped amount.

NAT: Snapshot work in progress on Windows ICMP API ping proxy. This commit adds new proxy code but doesn't hook it up yet.

NAT: We don't support Windows 2000, so use Windows ICMP API directly. We have been using runtime lookup for some functions and direct calls for some others which is inconsistent and proves "by construction" that the indirection is not necessary.

NAT: Start untangling Windows ICMP API code out of ip_icmp.c. Add new file ip_icmpwin.c and move icmp init/fini into it as-is.