unix/modsocket: Use type-checking mp_obj_get_int.

MP_OBJ_SMALL_INT_VALUE would give erroneous results, such as assertion
failures in the coverage build and other oddities like:

    >>> s = socket.socket()
    >>> s.recv(3.14)
    MemoryError: memory allocation failed, allocating 4235896656 bytes

Signed-off-by: Jeff Epler <jepler@gmail.com>

tests/extmod/socket_badconstructor.py

@@ -0,0 +1,22 @@
+# Test passing in bad values to socket.socket constructor.
+
+try:
+    import socket
+except:
+    print("SKIP")
+    raise SystemExit
+
+try:
+    s = socket.socket(None)
+except TypeError:
+    print("TypeError")
+
+try:
+    s = socket.socket(socket.AF_INET, None)
+except TypeError:
+    print("TypeError")
+
+try:
+    s = socket.socket(socket.AF_INET, socket.SOCK_RAW, None)
+except TypeError:
+    print("TypeError")