tests: add HTTP UNIX socket server testing support

The variable `$ipvnum` can now contain "unix" besides the integers 4
and 6 since the variable. Functions which receive this parameter
have their `$port` parameter renamed to `$port_or_path` to support a
path to the UNIX domain socket (as a "port" is only meaningful for TCP).

Signed-off-by: Peter Wu <peter@lekensteyn.nl>

tests/README

@@ -81,6 +81,9 @@ 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
+  location is 'http.sock'.
+
  1.4 Run
 
   'make test'. This builds the test suite support code and invokes the