pimd: Clean up pim RPF/NHT show commands

pimd: Clean up pim RPF/NHT show commands

Moved `show ip rpf A.B.C.D` command here from zebra, deprecated and aliased
to `show ip pim nexthop-lookup`.
Allow group to be optional in the lookup command. Only validate group if
source is ANY. Documented setting source via RP if not provided.
Added new output if ANY source + group lookup is performed and no
RP is found for the group. Updated output to include souce and
group for lookup.

Signed-off-by: Nathan Bahr <nbahr@atcorp.com>