'/proc/self/' is a good path which doesn't have stale time info but it
is only available for CONFIG_PROC_FS=y.

When CONFIG_PROC_FS=n, use argv0 instead of '/proc/self', use '/' for the
worst case.
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>