fix default behaviour of --spawn on X11-less TERMS

this ensures that whenever X11 is not present
and external commands like calling the editor
are used in combination with the --spawn parameter,
they are *not* called in a new xterm and forced to
run in the same thread as alot.

cf issue #460

alot/commands/globals.py

@@ -174,11 +174,14 @@ def __init__(self, cmd, stdin=None, shell=False, spawn=False,
             logging.debug('got: %s' % res)
             cmd, shell, self.in_thread = res
         # otherwise if spawn requested and X11 is running
-        elif spawn and 'DISPLAY' in os.environ:
-            term_cmd = settings.get('terminal_cmd', '')
-            logging.info('spawn in terminal: %s' % term_cmd)
-            termcmdlist = split_commandstring(term_cmd)
-            cmd = termcmdlist + cmd
+        elif spawn:
+            if 'DISPLAY' in os.environ:
+                term_cmd = settings.get('terminal_cmd', '')
+                logging.info('spawn in terminal: %s' % term_cmd)
+                termcmdlist = split_commandstring(term_cmd)
+                cmd = termcmdlist + cmd
+            else:
+                thread = False
 
         self.cmdlist = cmd
         self.stdin = stdin