This adds a "-q" option on the vvp command line and a vvp_set_quiet_flag()
function in the VVP library API. Setting this flag will cause all output
to standard output via MCD bit 0 to be suppressed. It will not prevent the
output being sent to a log file if the vvp "-l" option has been used, and
it will not affect output to the STDOUT file descriptor.