Changes

per-HNB GTP-U traffic counters via nft

Add optional feature: retrieve GTP-U traffic counters using nftables.

Add compile-time switch --enable-nftables, to build with/without
external dependency libnftables. Default is without, as before.

Add configure-time switch 'hnbgw' / 'nft-kpi' to enable use of nftables.
This requires osmo-hnbgw to be run with cap_net_admin.

The VTY config commands are always visible -- simplifies VTY testing.
Refuse to start osmo-hnbgw when requesting nft-kpi in the config but
when built without --enable-nftables.

When an hNodeB registers, set up nftables rules to count GTP-U packets
(UDP port 2152) to and from that hNodeB's address -- we are assuming
that it is the same address that Iuh is connecting from.

This is a "workaround" to get performance indicators per hNodeB, without
needing a UPF that supports URR.

This patch reads counters from the nftables response using "manual"
string parsing. See also Id4e7fa017c31945388a010d8581715d71482116b which
modifies this to full JSON parsing.

Tweaked-by: osmith
Related: SYS#6773
Depends: libosmocore I0df84b4bb8cb5d8434b735fa3a38e7f95be43e91
Change-Id: I35b7e97fd039e36633dfde1317170527c82f9f68