Check pager's executable before subprocess.Popen

Get the value of `PAGER` environment variable in case it's defined, check the executable, if it exists - use a pager, otherwise - print function.

Fixes: https://pagure.io/freeipa/issue/7746
Reviewed-By: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipalib/cli.py

@@ -49,7 +49,7 @@ import six
 from six.moves import input
 
 from ipalib.util import (
-    check_client_configuration, get_terminal_height, open_in_pager
+    check_client_configuration, get_pager, get_terminal_height, open_in_pager
 )
 
 if six.PY3:
@@ -721,9 +721,11 @@ class help(frontend.Local):
             self.buffer.append(unicode(string))
 
         def write(self):
-            if self.buffer_length > get_terminal_height():
+            pager = get_pager()
+
+            if pager and self.buffer_length > get_terminal_height():
                 data = "\n".join(self.buffer).encode("utf-8")
-                open_in_pager(data)
+                open_in_pager(data, pager)
             else:
                 try:
                     for line in self.buffer: