qtvcp: mdi_line use halcmd for net command instead of halmodule

Increases functionality while decreasing confusion, bonus of more robust 
error messages.

Author: snowgoer540

docs/src/gui/qtvcp-widgets.adoc

@@ -1892,12 +1892,10 @@ program or access the feature:
   link:http://linuxcnc.org/docs/devel/html/ladder/classic-ladder.html[ClassicLadder GUI] if the ClassicLadder realtime HAL component was loaded by the machine's config files
 * `PREFERENCE` - Loads the preference file onto the gcodeEditor
 * `CLEAR HISTORY` - Clears the MDI History
-* `net` - Connects a pin to a signal. An error will result if
-  the command is unsuccessful. Running LinuxCNC from terminal
-  may help determine the root cause as error messages from hal_lib.c
-  will be displayed there.
-** _Syntax_: `net <pin name> <signal name>`
-** __Example__: `net motion.jog-stop plasmac:jog-inhibit`
+* `net` - See link:http://linuxcnc.org/docs/devel/html/man/man1/halcmd.1.html#COMMANDS[halcmd net COMMAND].
+  An error will result if the command is unsuccessful.
+** _Syntax_: `net <signal name> <pin name>`
+** __Example__: `net plasmac:jog-inhibit motion.jog-stop`
 * `setp` - Sets the value of a pin or a parameter. Valid values depend
   on the object type of the pin or parameter. An error will result if
   the data types do not match or the pin is connected to a signal.

lib/python/qtvcp/widgets/mdi_line.py

@@ -19,6 +19,8 @@
 import hal
 import time
 
+import subprocess
+
 from PyQt5.QtWidgets import QLineEdit, QApplication
 from PyQt5.QtCore import Qt, QEvent, pyqtProperty
 
@@ -204,14 +206,19 @@ def unlinkp(self, unlinkpString):
 
     def net(self, netString):
         arguments = len(netString.lower().replace('net',' ').split())
-        if arguments == 2:
-            halpin, signal = netString.lower().replace('net',' ').split()
+        if arguments >= 2:
+            args = ['halcmd', 'net']
+            split = netString.lower().replace('net',' ').split()
+            for arg in split:
+                args.append(arg)
         else:
-            ACTION.SET_ERROR_MESSAGE('NET ERROR:\nnet requires 2 arguments, {} given\n'.format(arguments))
+            ACTION.SET_ERROR_MESSAGE('NET ERROR:\nnet requires at least 2 arguments, {} given\n'.format(arguments))
             return
-        reply = hal.connect(halpin, signal)
-        if reply:
-            ACTION.SET_ERROR_MESSAGE('NET ERROR:\nFailed to link pin "{}" to signal "{}"\n'.format(halpin, signal))
+        reply = subprocess.Popen(args, stderr=subprocess.PIPE)
+        stdout, stderr = reply.communicate()
+        if stderr:
+            error = stderr.decode().replace('<commandline>:0:', '').strip()
+            ACTION.SET_ERROR_MESSAGE('NET ERROR:\n{}\n'.format(error))
 
     def spindle_inhibit(self, state):
         self.spindleInhibit = state