GTP mockup: allow single GTP devices in mockup mode

Drop the global GTP mockup mode, and instead allow single GTP devices to
be mockup devices.

Do not always open the mnl_socket, but only open it when an actual
non-mockup GTP device is being opened. (In the presence of non-mockup
GTP devices, this still happens directly upon program startup.)

The GTP mockup was first introduced for VTY tests during 'make check'.
So far, in mockup mode, all GTP tunnel actions were simply cut short,
and no tunnels were ever listed as active.

TTCN3 tests do query osmo-upf to list currently active tunnels, so the
GTP mockup fails these tests. To allow running TTCN3 tests without
cap_net_admin privileges, instead of a global GTP mockup mode, allow
creating single GTP device(s) in mockup mode, which then also list their
(fake) active GTP tunnels.

Also useful in a lab environment: send PFCP commands to osmo-upf and get
a listing of GTP tunnels that would have been active.

So far, osmo-upf always uses only the first GTP device configured, but
if we add support for multiple GTP devices, this patch allows putting
only single devices in mockup mode.

Related: SYS#5599
Change-Id: Ic09a5ccea24086eb04f46e6af669668e5fade752
---
M doc/examples/osmo-upf/osmo-upf-mockup.cfg
M include/osmocom/upf/upf.h
M include/osmocom/upf/upf_gtp.h
M src/osmo-upf/osmo_upf_main.c
M src/osmo-upf/up_gtp_action.c
M src/osmo-upf/upf.c
M src/osmo-upf/upf_gtp.c
M src/osmo-upf/upf_vty.c
M tests/upf.vty
9 files changed, 82 insertions(+), 66 deletions(-)