vppinfra: bihash add-but-do-not-overwrite semantics
vppinfra: bihash add-but-do-not-overwrite semantics

If is_add=2, fail w/ return value -2 if the key exists instead of
overwriting the (key,value) pair.

Type: feature

Change-Id: I00a3c194a381c68090369c31d6c6f9870cfe0a62
Signed-off-by: Dave Barach <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@barachs.net>

chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp