Fix TFTP problems with large files.

TFTP fails with a timeout when the transfer exceeds 128K blocks.
The block field in an ACK packet is 16 bits, so large transfers
need to deal with this field wrapping. Testing failure meant
that it worked for the first wrap, but not for subsequent ones.

Having fixed the block number problem, file size calculations
accidentally being done in 32 bits not 64 bits then broke
transfers larger than 2G or maybe 4G.

The first problem was probably introduced in commit
ebef27f321 and has not appeared
in a stable release. The second appears to have been there forever.
Clearly, nobody is mad enough to transfer multi-gigabyte files
over TFTP.

Thanks to Jean-François JUBLIN for spotting this and supplying
useful packet dumps that made it easy to diagnose.

src/forward.c

@@ -3106,7 +3106,7 @@ static void free_frec(struct frec *f)
 /* Impose an absolute
    limit of 4*TIMEOUT before we wipe things (for random sockets).
    If force is set, always return a result, even if we have
-   to allocate above the limit, and don'y free any records.
+   to allocate above the limit, and don't free any records.
    This is set when allocating for DNSSEC to avoid cutting off
    the branch we are sitting on. */
 static struct frec *get_new_frec(time_t now, struct server *master, int force)