vcd: Output timestamp only once per change.

Avoid writing a new timestamp for every changed signal if multiple
signals change state simultaneously.  Also, keep signal transitions
on the same line with their timestamp to make the output easier to
inspect in a text editor.  (VCD does not care whether newlines or
spaces are used to separate tokens.)
vcd: Remove bogus $dumpvars and $dumpoff commands.

These commands are superfluous and do not seem to make sense in
the context they were used.  Also, $dumpvars was missing an $end,
and $dumpoff was used without any content.
Add librevisa SCPI backend.
scpi_visa.c: Minor cosmetics.