Increase maximum -b value to 65507

Increase maximum -b value to 65507

Correct definitions: maximum theoretical IPv4 packet size and minimum
IPv4 header size (previously probably IPv6 size was used).

Because fping does not know ahead whether address is IPv4 or IPv6 assume
IPv4. Previously fping allowed only 65488, but real maximum for IPv4 on
Linux is 65507 (IPv6 would allow 65527 follow the current approach and
allow smaller value than reachable for IPv6).

Update test affected by this change. While at this, test 65508 (one
above the limit) instead of 65509.

Signed-off-by: Petr Vorel <pvorel@suse.cz>