Handle proc fs as usual if mounted somewhere else than /proc

For /proc, we need directory mode "read only",
but we only checked for fstype being "proc" and not for the path.
This meant that BenchExec would mount proc mounts
in other places than /proc as read only and ignore the directory mode
specified by the user.
This creates problems if the parent directory is hidden, for example.

Fixes #988

benchexec/container.py

@@ -582,7 +582,7 @@ def determine_directory_mode(dir_modes, path, fstype=None):
     From a high-level mapping of desired directory modes, determine the actual mode
     for a given directory.
     """
-    if fstype == b"proc":
+    if path == b"/proc" and fstype == b"proc":
         # proc is necessary for the grandchild to read PID, will be replaced later.
         return DIR_READ_ONLY
     if util.path_is_below(path, b"/proc"):