This patch fixes a number of problems associated with the various dumpers.
1. It catches the problem uncovered in pr1809904 and prevents the core
dumps from happening. It prints a warning message if you try to change
the file after the program has started dumping data to the file.
2. It makes all the dumpers work in the same manner. Before the VCD
$dumpfile was processed at compile time, but the LXT/LXT2 versions
did this at run time. The correct place for this is the run time so
that you can pass the task a calculated name.
3. All the dumpers use common compiletf routines located in vcd_priv.c
4. Make the LXT/LXT2 $dumpfile commands only get the file name and
let $dumpvars actually open the file. This matches the VCD code.
5. Fix the $dumpfile code to allow calculated file names.
6. Make dumpvars without a scope/variable match all toplevel modules
not just the toplevel module the $dumpvars was located in. This
now matches the standard (2001).
7. Simplify the no dumper code (vcdoff.c) and add missing functions.
8. Cleanup the code and messages.
9. vvp can take -none for no dumper.
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