code/docs: Use Unix rather than UNIX to avoid use of the trademark

Use Unix when generically writing about Unix based systems as UNIX is
the trademark and should only be used in a particular product's name.

tests/README

@@ -81,7 +81,7 @@ The cURL Test Suite
   machine, or just move the servers in case you have local services on any of
   those ports.
 
-  The HTTP server supports listening on a UNIX domain socket, the default
+  The HTTP server supports listening on a Unix domain socket, the default
   location is 'http.sock'.
 
  1.4 Run