patch 8.0.0294: argument list is not stored correctly in a session file

Problem:    Argument list is not stored correctly in a session file.
            (lgpasquale)
Solution:   Use "$argadd" instead of "argadd". (closes #1434)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 1a10bb0..f93c766 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -11790,7 +11790,7 @@
 		    s = buf;
 		}
 	    }
-	    if (fputs("argadd ", fd) < 0
+	    if (fputs("$argadd ", fd) < 0
 		    || ses_put_fname(fd, s, flagp) == FAIL
 		    || put_eol(fd) == FAIL)
 	    {