improve selectsave handling for Windows

If there were outdated savefiles encountered during
startup, each individual one was getting a wait_synch
that required a <return> even though a message window
wasn't being used at that point.

Allow suppression of the individual per-file wait_synch()
calls on Windows, so that a single one can be done once
the selectsave processing is overwith.

This was a little messy because an indicator had to flow
down through validate(), uptodate(), etc.

There shouldn't be any change in how things behave on
any non-Windows platforms.

include/hack.h

@@ -830,6 +830,7 @@ typedef struct {
 #define UTD_CHECKFIELDCOUNTS           0x02
 #define UTD_SKIP_SANITY1               0x04
 #define UTD_SKIP_SAVEFILEINFO          0x08
+#define UTD_WITHOUT_WAITSYNCH_PERFILE  0x10
 
 #define ENTITIES 2
 struct valuable_data {