Windows: flush shell notifications when dismounting with /q

[amail80]

Follow-up to #1658. That fix gates the synchronous SHChangeNotify on Silent,
but the immediate process exit is caused by Quit, and the two are set by
different command line switches:

/s, /silent  ->  Silent = TRUE     (suppresses the user interface)
/q, /quit    ->  Quit   = TRUE     (exit after processing command line)

/q does not imply /s, so VeraCrypt /dismount X: /q never reaches the
SHCNF_FLUSH branch and the ghost drive letter reported in #337 and #1426 can
still appear. This is what the process actually terminates on
(src/Mount/Mount.c):

if (Quit)
{
    if (TaskBarIconMutex == NULL)
    {
        SignalExitCode (exitCode);
        exit (exitCode);
    }

Observable without hitting the race: the same Silent check also selects how
the unmount runs, so VeraCrypt /d X: /q displays the wait dialog, which only
happens when Silent is FALSE - confirming the flush branch is skipped on this
path. Whether a ghost letter then appears depends on Explorer processing the
asynchronous notification before the process exits, so it is timing dependent;
the missing flush is not.

Quit is defined in Mount.c only, while half of the fix lives in Dlgcode.c,
which is also linked into Format and ExpandVolume - and ExpandVolume defines its
own Quit, so the symbol cannot simply be moved. This adds a small shared flag
instead:

• src/Common/Dlgcode.c/.h - BOOL ProcessExitsAfterCommand
• src/Mount/Mount.c - set next to each Quit = TRUE; in ExtractCommandLine()
• both guards become if (Silent || ProcessExitsAfterCommand)

Silent is kept in the condition, so existing behaviour is unchanged; the patch
is purely additive.

Also reported on SourceForge as ticket #629.