To avoid two phones picking mismatching TCH pchans, never pick TCH/F on dynamic
TS in osmo-nitb.
Add gsm_network flag dyn_ts_allow_tch_f, set to true by default in
gsm_network_init().
Set this flag to false in osmo-nitb's main().
See http://osmocom.org/issues/1778
Reasoning about ways to solve this:
* a compile time switch doesn't work because libbsc is first compiled and then
linked to both osmo-nitb and osmo-bsc.
* we could test net->bsc_api == msc_bsc_api(), but I have the so-called MSC
split waiting on branch sysmocom/cscn, which will result in msc_bsc_api() not
being linked in the osmo-bsc binary.
* have a function am_i_nitb() with different implementations in osmo-nitb and
osmo-bsc, but then we'd need to add implementations to all tests and other
binaries linking lchan_alloc().
* have a flag in struct bsc_api, but so far there are only function pointers
there.
Having a "global" flag in gsm_network allows to add a VTY command in case we
decide to keep this feature (#1781), has no linking implications and is nicely
explicit.
Tested that osmo-bsc still picks TCH/F on dyn TS indirectly, since I have no
standalone MSC available: when compiling osmo-nitb with the line that sets
dyn_ts_allow_tch_f = false commented out, TCH/F is picked as described in
OS#1778; and by printf-verifying that dyn_ts_allow_tch_f == true in osmo-bsc
main(), only osmo-nitb should have TCH/F disabled.
Related: OS#1778, OS#1781
Change-Id: If7e4797a72815fc6e2bbef27756ea5df69f4bde7
5f0c71b7d5