ip/iptunnel: Extend TOS syntax

ip/iptunnel: Extend TOS syntax

The 'inherit/STRING' or 'inherit/00..ff' syntax indicates that the
TOS field of tunneled packets should be copied from the original IP
header, but for non-IP packets the value STRING or 00..ff should be
used instead. (This syntax is already used by 'ip tunnel show'.)

Also clarify the man page and the command usage text (particularly
that the TOS is not specified as a decimal number).

Signed-off-by: David Ward <david.ward@ll.mit.edu>