bgpd: Adjust terminology related to DSCP

bgpd: Adjust terminology related to DSCP

The default DSCP used for BGP connections is CS6. The DSCP value is
not part of the TCP header.

When setting the IP_TOS or IPV6_TCLASS socket options, the argument
is not the 6-bit DSCP value, but an 8-bit value for the former IPv4
Type of Service field or IPv6 Traffic Class field, respectively.

Fixes: 425bd64be847 ("bgpd: Allow bgp to control the DSCP session TOS value")
Signed-off-by: David Ward <david.ward@ll.mit.edu>