Since the IETF has redefined the meaning of the tos field to accommodate
a set of differentiated services, set IPTOS_PREC_* macros using
IPTOS_DSCP_* macro definitions.

While here, add IPTOS_DSCP_VA macro according to RFC 5865.

Differential Revision: https://reviews.freebsd.org/D3119
Reviewed by: gnn