sysprofd: add API to get a FD for a file in proc

This is useful for optimized parsing of proc files such as necessary by
the memory source to seek() back to the start (instead of re-opening a
new file).

src/helpers.h

@@ -39,5 +39,7 @@ gboolean helpers_perf_event_open (GVariant     *options,
 gboolean helpers_get_proc_file   (const gchar  *path,
                                   gchar       **contents,
                                   gsize        *len);
+gboolean helpers_get_proc_fd     (const gchar  *path,
+                                  gint         *out_fd);
 
 G_END_DECLS