Change the name of the xShmClose VFS method to xShmUnmap, everywhere.

FossilOrigin-Name: c2d27cf51d33e6f38bab37008d39074051f75274

src/os_unix.c

@@ -3607,6 +3607,9 @@ static void unixShmBarrier(
 /*
 ** Close a connection to shared-memory.  Delete the underlying 
 ** storage if deleteFlag is true.
+**
+** If there is no shared memory associated with the connection then this
+** routine is a harmless no-op.
 */
 static int unixShmUnmap(
   sqlite3_file *fd,               /* The underlying database file */