input/vcd: Avoid truncation of shift to 32 bits.

(parse_contents): Cast constant 1 to uint64_t before shifting left.
Also fix whitespace errors.
input/vcd: Remove debug output from inner loops.

(parse_contents): Do not call sr_dbg() on every signal change.
This would be excessive even for sr_spew().
(read_until): Do not call ftell() just to be able to show some
number in a debug message later on.
output/vcd: Extend output stream to last sample.

(receive): Write a dummy timestamp after the last sample
to indicate the length of the sampled stream.