dtc: Make dt_from_blob() open its own input file, like the other input formats [resend]

Currently, main() has a variable for the input file. It used to be that main() would open the input based on command line arguments before passing it to the dt_from_*() function. However, only dt_from_blob() uses this. dt_from_source() opens its own file, and dt_from_fs() interprets the argument

dtc: Make dt_from_blob() open its own input file, like the other input formats

Currently, main() has a variable for the input file. It used to be that main() would open the input based on command line arguments before passing it to the dt_from_*() function. However, only dt_from_blob() uses this. dt_from_source() opens its own file, and dt_from_fs() interprets the argument