src: proto: support DF, LE PHB, VA for DSCP

src: proto: support DF, LE PHB, VA for DSCP

Add a couple of aliases for well-known DSCP values.

As per RFC 4594, add "df" as an alias of "cs0" with 0x00 value.

As per RFC 5865, add "va" for VOICE-ADMIT with 0x2c value.

As per RFC 8622, add "lephb" for Lower-Effort Per-Hop Behavior with 0x01 value.

tc-cake(8) in diffserv8 mode would benefit from having "lephb" defined since
it corresponds to "Tin 0".

https://www.iana.org/assignments/dscp-registry/dscp-registry.xhtml

Signed-off-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>