tc: gred: Adopt the term VQ in the command syntax and output

tc: gred: Adopt the term VQ in the command syntax and output

In the GRED kernel source code, both of the terms "drop parameters"
(DP) and "virtual queue" (VQ) are used to refer to the same thing.
Each "DP" is better understood as a "set of drop parameters", since
it has values for limit, min, max, avpkt, etc. This terminology can
result in confusion when creating a GRED qdisc having multiple DPs.
Netlink attributes and struct members with the DP name seem to have
been left intact for compatibility, while the term VQ was otherwise
adopted in the code, which is more intuitive.

Use the VQ term in the tc command syntax and output (but maintain
compatibility with the old syntax).

Rewrite the usage text to be concise and similar to other qdiscs.

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