Fix binfmt_misc check (#1449)

We require binfmt_misc to run programs of foreign architectures (e.g.
armhf) with QEMU. This is set up by default in most distributions, but
in some (e.g. Alpine, Void) it needs to be configured manually (see
 the troubleshooting page in the wiki).

We have a check in place, which points to that troubleshooting wiki
page. However, the check was flawed, because we assumed the binfmt_misc
folder would not exist.

Thanks to @fxkrait for making the fix and for testing it!

pmb/helpers/other.py

@@ -68,7 +68,7 @@ def check_binfmt_misc(args):
     /proc/sys/fs/binfmt_misc/ exists. If it exists, then do nothing.
     Otherwise, raise an exception pointing to user to the Wiki.
     """
-    path = "/proc/sys/fs/binfmt_misc/"
+    path = "/proc/sys/fs/binfmt_misc/status"
     if os.path.exists(path):
         return