proxy: make path_sim values configurable via VTY

Move the RF path loss and default reported ToA/CI for forwarded
bursts ('path-loss'/'nominal-toa256'/'nominal-ci' under 'proxy'),
and each endpoint's nominal Tx power ('tx-power' under 'ep'), out
of path_sim.c's compile-time constants and into new VTY commands.

The global values are held in a new opaque struct path_sim_cfg,
talloc-allocated by path_sim_cfg_alloc() and exposed only through
path_sim_cfg_{get,set}_*() accessors, keeping struct proxy_ctx down to
a single pointer instead of exposing path_sim internals.  tx_power is
per-endpoint and lives directly on struct proxy_trx.  Both are applied
when a channel's path_sim_state is (re)initialized.

Change-Id: I969e9df1ab394e631da3da9b3044fab1446db40e
Related: OS#6672
---
M proxy/include/osmocom/proxy/path_sim.h
M proxy/include/osmocom/proxy/proxy.h
M proxy/include/osmocom/proxy/trx.h
M proxy/src/burst_fwd.c
M proxy/src/ctrl_cmd.c
M proxy/src/path_sim.c
M proxy/src/proxy.c
M proxy/src/trx.c
M proxy/src/vty.c
9 files changed, 194 insertions(+), 33 deletions(-)