POSIX like Sergei Organov recommended.
+ A task owner lets that rtems_set_private_env() will be
called twice or more times.
+ chroot() can be called without a previous
rtems_set_private_env(); (transpanrently)
+ The second call of rtems_set_private_env() makes a internal
chroot("/") into global imfs_root.
+ chroot() runs like chdir() without a previous chdir("/") with
the global root.
+ The current directory can be in a wrong place like Linux and
many other Unices.