ServerTransportListenAddr validation should validate that transport-name is well-formed

Someone put in his torrc:

ServerTransportListenAddr obfs2,obfs3 0.0.0.0:56831 0.0.0.0:56832

inspired by the format of ServerTransportPlugin. Unfortunately, this is not the correct way to use ServerTransportListenAddr. The correct way is:

ServerTransportListenAddr obfs2 0.0.0.0:56831
ServerTransportListenAddr obfs3 0.0.0.0:56832

We should at least validate that the first argument of the line is a pluggable transport name (C-identifier) to avoid stuff like "obfs2,obfs3".