py/stream: Move ad-hoc ioctl constants to stream.h and rename them.

The constants MP_IOCTL_POLL_xxx, which were stmhal-specific, are moved
from stmhal/pybioctl.h (now deleted) to py/stream.h.  And they are renamed
to MP_STREAM_POLL_xxx to be consistent with other such constants.

All uses of these constants have been updated.
diff --git a/py/stream.h b/py/stream.h
index 94002e2..01199ab 100644
--- a/py/stream.h
+++ b/py/stream.h
@@ -42,6 +42,12 @@
 #define MP_STREAM_GET_DATA_OPTS (8)  // Get data/message options
 #define MP_STREAM_SET_DATA_OPTS (9)  // Set data/message options
 
+// These poll ioctl values are compatible with Linux
+#define MP_STREAM_POLL_RD  (0x0001)
+#define MP_STREAM_POLL_WR  (0x0004)
+#define MP_STREAM_POLL_ERR (0x0008)
+#define MP_STREAM_POLL_HUP (0x0010)
+
 // Argument structure for MP_STREAM_SEEK
 struct mp_stream_seek_t {
     mp_off_t offset;