patch 8.1.0744: compiler warnings for signed/unsigned strings

Problem:    Compiler warnings for signed/unsigned strings.
Solution:   A few more type cast fixes.
diff --git a/src/if_py_both.h b/src/if_py_both.h
index 165c847..85141e1 100644
--- a/src/if_py_both.h
+++ b/src/if_py_both.h
@@ -410,6 +410,12 @@
 }
 
     static int
+msg_wrapper(char *text)
+{
+    return msg((char_u *)text);
+}
+
+    static int
 write_output(OutputObject *self, PyObject *string)
 {
     Py_ssize_t	len = 0;
@@ -421,7 +427,7 @@
 
     Py_BEGIN_ALLOW_THREADS
     Python_Lock_Vim();
-    writer((writefn)(error ? emsg : msg), (char_u *)str, len);
+    writer((writefn)(error ? emsg : msg_wrapper), (char_u *)str, len);
     Python_Release_Vim();
     Py_END_ALLOW_THREADS
     PyMem_Free(str);
@@ -634,7 +640,7 @@
     else if (msg_list != NULL && *msg_list != NULL)
     {
 	int	should_free;
-	char_u	*msg;
+	char	*msg;
 
 	msg = get_exception_string(*msg_list, ET_ERROR, NULL, &should_free);
 
@@ -644,7 +650,7 @@
 	    return -1;
 	}
 
-	PyErr_SetVim((char *) msg);
+	PyErr_SetVim(msg);
 
 	free_global_msglist();
 
@@ -3483,13 +3489,13 @@
     static int
 set_option_value_err(char_u *key, int numval, char_u *stringval, int opt_flags)
 {
-    char_u	*errmsg;
+    char	*errmsg;
 
     if ((errmsg = set_option_value(key, numval, stringval, opt_flags)))
     {
 	if (VimTryEnd())
 	    return FAIL;
-	PyErr_SetVim((char *)errmsg);
+	PyErr_SetVim(errmsg);
 	return FAIL;
     }
     return OK;