[tcp] Reduce path MTU to 1280 bytes

The path MTU is currently hardcoded to 1460 bytes, which fails to
allow space for TCP options.  Sending a maximum-sized datagram (which
is viable when using HTTP POST) will therefore fail since the Ethernet
MTU will be exceeded.

Reduce the hardcoded path MTU to produce a maximum datagram of 1280
bytes, which is the size required of data link layers by IPv6.  It is
a reasonable assumption that all intermediary data link layers will be
able to convey this packet without fragmentation, even for IPv4.

Note that this reduction has a minimal impact upon download
throughput, since it affects only the transmit data path.

Originally-fixed-by: Suresh Sundriyal <ssundriy@vmware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>