patch 8.0.1554: custom plugins loaded with --clean

Problem:    Custom plugins loaded with --clean.
Solution:   Do not include the home directory in 'runtimepath'.
diff --git a/src/option.c b/src/option.c
index 9662039..1b51447 100644
--- a/src/option.c
+++ b/src/option.c
@@ -3345,9 +3345,10 @@
  * Initialize the options, first part.
  *
  * Called only once from main(), just after creating the first buffer.
+ * If "clean_arg" is TRUE Vim was started with --clean.
  */
     void
-set_init_1(void)
+set_init_1(int clean_arg)
 {
     char_u	*p;
     int		opt_idx;
@@ -3554,6 +3555,24 @@
      */
     set_options_default(0);
 
+#ifdef CLEAN_RUNTIMEPATH
+    if (clean_arg)
+    {
+	opt_idx = findoption((char_u *)"runtimepath");
+	if (opt_idx >= 0)
+	{
+	    options[opt_idx].def_val[VI_DEFAULT] = (char_u *)CLEAN_RUNTIMEPATH;
+	    p_rtp = (char_u *)CLEAN_RUNTIMEPATH;
+	}
+	opt_idx = findoption((char_u *)"packpath");
+	if (opt_idx >= 0)
+	{
+	    options[opt_idx].def_val[VI_DEFAULT] = (char_u *)CLEAN_RUNTIMEPATH;
+	    p_pp = (char_u *)CLEAN_RUNTIMEPATH;
+	}
+    }
+#endif
+
 #ifdef FEAT_GUI
     if (found_reverse_arg)
 	set_option_value((char_u *)"bg", 0L, (char_u *)"dark", 0);