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/os_unix.h b/src/os_unix.h
index 695affa..4146529 100644
--- a/src/os_unix.h
+++ b/src/os_unix.h
@@ -366,11 +366,14 @@
 
 #ifdef VMS
 # define DFLT_RUNTIMEPATH      "sys$login:vimfiles,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,sys$login:vimfiles/after"
+# define CLEAN_RUNTIMEPATH      "$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after"
 #else
 # ifdef RUNTIME_GLOBAL
 #  define DFLT_RUNTIMEPATH	"~/.vim," RUNTIME_GLOBAL ",$VIMRUNTIME," RUNTIME_GLOBAL "/after,~/.vim/after"
+#  define CLEAN_RUNTIMEPATH	RUNTIME_GLOBAL ",$VIMRUNTIME," RUNTIME_GLOBAL "/after"
 # else
 #  define DFLT_RUNTIMEPATH	"~/.vim,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,~/.vim/after"
+#  define CLEAN_RUNTIMEPATH	"$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after"
 # endif
 #endif