Revert spawn's splitting, blacklist from doc

Blacklist spawn from getting the no_replace_variables doc note.

doc/help/commands.asciidoc

@@ -696,7 +696,7 @@ You can use the `{url}` and `{url:pretty}` variables here which will get replace
 
 [[spawn]]
 === spawn
-Syntax: +:spawn [*--userscript*] [*--verbose*] [*--detach*] 'cmdline' ['cmdline' ...]+
+Syntax: +:spawn [*--userscript*] [*--verbose*] [*--detach*] 'cmdline'+
 
 Spawn a command in a shell.
 
@@ -715,6 +715,9 @@ Note the `{url}` and `{url:pretty}` variables might be useful here. `{url}` gets
 * +*-v*+, +*--verbose*+: Show notifications when the command started/exited.
 * +*-d*+, +*--detach*+: Whether the command should be detached from qutebrowser.
 
+==== note
+* This command does not split arguments after the last argument and handles quotes literally.
+
 [[stop]]
 === stop
 Stop loading in the current/[count]th tab.

qutebrowser/browser/commands.py

@@ -991,8 +991,9 @@ class CommandDispatcher:
         finally:
             self._tabbed_browser.setUpdatesEnabled(True)
 
-    @cmdutils.register(instance='command-dispatcher', scope='window')
-    def spawn(self, *cmdline, userscript=False, verbose=False, detach=False):
+    @cmdutils.register(instance='command-dispatcher', scope='window',
+                       maxsplit=0, no_replace_variables=True)
+    def spawn(self, cmdline, userscript=False, verbose=False, detach=False):
         """Spawn a command in a shell.
 
         Note the `{url}` and `{url:pretty}` variables might be useful here.
@@ -1011,7 +1012,14 @@ class CommandDispatcher:
             detach: Whether the command should be detached from qutebrowser.
             cmdline: The commandline to execute.
         """
-        cmd, *args = cmdline
+        try:
+            cmd, *args = shlex.split(cmdline)
+        except ValueError as e:
+            raise cmdexc.CommandError("Error while splitting command: "
+                                      "{}".format(e))
+
+        args = runners.replace_variables(self._win_id, args)
+
         log.procs.debug("Executing {} with args {}, userscript={}".format(
             cmd, args, userscript))
         if userscript:

scripts/dev/src2asciidoc.py

@@ -240,7 +240,7 @@ def _get_command_doc_notes(cmd):
         Strings which should be added to the docs.
     """
     if (cmd.maxsplit is not None or cmd.no_cmd_split or
-            cmd.no_replace_variables):
+            cmd.no_replace_variables and cmd.name != "spawn"):
         yield ""
         yield "==== note"
         if cmd.maxsplit is not None:
@@ -249,7 +249,7 @@ def _get_command_doc_notes(cmd):
         if cmd.no_cmd_split:
             yield ("* With this command, +;;+ is interpreted literally "
                    "instead of splitting off a second command.")
-        if cmd.no_replace_variables:
+        if cmd.no_replace_variables and cmd.name != "spawn":
             yield r"* This command does not replace variables like +\{url\}+."