Don't put VQuiet in VFlags; that's reserved for user-toggles.

Since VQuiet is set programmatically, it's inappropriate
for it to be propagated to the command line.  But unfortunately,
if it was set, we were accidentally using the fancy flag format.
This patch moves VQuiet to its own field in Verbosity so we
don't get confused.

Fixes #4393.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>