Some cleanup in tcp_respond() in preparation for another change:
- Reorder variables by size
- Move initializer closer to where it is used
- Remove unneeded variable

As reported on the transport@ and current@ mailing lists, the FreeBSD TCP
stack is not compliant with RFC 7323, which requires that TCP stacks send
a timestamp option on all packets (except, optionally, RSTs) after the
session is established.

This patch adds that support. It also adds a TCP signature option to the
packet, if appropriate.

Don't reuse the source mbuf in tcp_respond() if it is not writable.

Not all mbufs passed up from device drivers are M_WRITABLE().  In
particular, the Chelsio T4/T5 driver uses a feature called "buffer
packing" to receive multiple frames in a single receive buffer.  The mbufs
to receive multiple frames in a single receive buffer.  The mbufs for
these frames all share the same external storage so are treated as
read-only by the rest of the stack when multiple frames are in flight.
Previously tcp_respond() would blindly overwrite read-only mbufs when
INVARIANTS was disabled or panic with an assertion failure if INVARIANTS
was enabled.  Note that the new case is a bit of a mix of the two other
cases in tcp_respond().  The TCP and IP headers must be copied explicitly
into the new mbuf instead of being inherited (similar to the m == NULL
case), but the addresses and ports must be swapped in the reply (similar
to the m != NULL case).