unix: Rename "microsocket" module to "usocket".

Per new conventions, we'd like to consistently use "u*" naming conventions
for modules which don't offer complete CPython compatibility, while offer
subset or similar API.

examples/unix/http-client.py

@@ -1,5 +1,5 @@
 try:
-    import microsocket as _socket
+    import usocket as _socket
 except:
     import _socket
 

examples/unix/http-server.py

@@ -1,5 +1,5 @@
 try:
-    import microsocket as socket
+    import usocket as socket
 except:
     import socket