stream/dvbin: detect ZAP channel configs looking like VDR configs

While ZAP config files should contain strings in most fields,
their field number in the DVB-T / ISDBT case matches the number of fields
for VDR config files, and some channel list writers (namely, dvbv5-scan)
may insert "0" in unused string fields.
To disentangle such config files from real VDR config files,
add a check for the "INVERSION_" field which should always be filled.