Change the unix VFS so that it ignores all but the least-significant bit

of the syncDir flag to xDelete.  Add an assert to prove that the core only
ever uses that one bit.

FossilOrigin-Name: e75fd3b27423272b988921ac0e272f9600818b8c

src/os.c

@@ -151,6 +151,7 @@ int sqlite3OsOpen(
 }
 int sqlite3OsDelete(sqlite3_vfs *pVfs, const char *zPath, int dirSync){
   DO_OS_MALLOC_TEST(0);
+  assert( dirSync==0 || dirSync==1 );
   return pVfs->xDelete(pVfs, zPath, dirSync);
 }
 int sqlite3OsAccess(