Add additional VFS methods to retrieve system call pointers and to get a

list of all changeable system calls.

FossilOrigin-Name: 38558363494e3a736dcb091dd859e76b7ccd78b0

src/os_win.c

@@ -2766,6 +2766,8 @@ int sqlite3_os_init(void){
     winGetLastError,     /* xGetLastError */
     winCurrentTimeInt64, /* xCurrentTimeInt64 */
     0,                   /* xSetSystemCall */
+    0,                   /* xGetSystemCall */
+    0,                   /* xNextSystemCall */
   };
 
 #ifndef SQLITE_OMIT_WAL