Issue #13854: Properly handle non-integer, non-string arg to SystemExit

Previously multiprocessing only expected int or str.  It also wrongly
used an exit code of 1 when the argument was a string instead of zero.
diff --git a/Lib/multiprocessing/process.py b/Lib/multiprocessing/process.py
index 0697e74..482ea0a 100644
--- a/Lib/multiprocessing/process.py
+++ b/Lib/multiprocessing/process.py
@@ -262,12 +262,12 @@
         except SystemExit, e:
             if not e.args:
                 exitcode = 1
-            elif type(e.args[0]) is int:
+            elif isinstance(e.args[0], int):
                 exitcode = e.args[0]
             else:
-                sys.stderr.write(e.args[0] + '\n')
+                sys.stderr.write(str(e.args[0]) + '\n')
                 sys.stderr.flush()
-                exitcode = 1
+                exitcode = 0 if isinstance(e.args[0], str) else 1
         except:
             exitcode = 1
             import traceback