Fix compiler warnings (original patches from Enrico Tröger)

Add missing include for geteuid() missing declaration
xtm_process_window_show_swap_usage() in process-window.
Fix various compiler warnings from main.c task-manager.c and
app-manager.c.

src/process-window.c

@@ -11,6 +11,10 @@
 #include <config.h>
 #endif
 
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
 #include <glib-object.h>
 #include <glib/gi18n.h>
 #include <gtk/gtk.h>