T: improve to_vcd tracer

Make it compatible with 'template' gtwave files as found in openair's
repository.

The type VCD_NAME was added to the VCD IDs in T_messages.txt.
The database processing code was adapted to use it.
The tracer to_vcd was adapted to use it and render the produced vcd
output compatible with the 'template' gtkwave files.

When running to_vcd, do not forget to pass -vcd to use this feature.