Changes

osmo-bts-omldummy: properly report NM_ATT_IPACC_SUPP_FEATURES

osmo-bts-omldummy is used as the OML backend in the ttcn3-bsc-test.
The common OML code encodes bts->support.* and trx->support.* fields
verbatim into the IPA Supported Features IE (NM_ATT_IPACC_SUPP_FEATURES)
in Get Attributes Responses.  Since bts_model_init() and
bts_model_trx_init() never initialized these fields, they were reported
as all-zeros to osmo-bsc.

osmo-bsc now inspects the supported channel modes before activating a
channel.  An all-zero chan_modes value (present but no bits set) causes
osmo-bsc to reject every speech mode, breaking all assignment-related
TTCN-3 test cases.

Initialize ciphers, gprs_codings, freq_bands, chan_types and chan_modes
to sensible values, matching what osmo-bts-virtual does, so that
osmo-bts-omldummy reports its capabilities correctly.

Change-Id: I7fe83f78c829c300e70a59509847b815a77974d7
Fixes: 0978d1df ("oml: implement handling of NM_ATT_IPACC_SUPP_FEATURES")
Related: OS#6324