File terminal is a service that provides a terminal-session interface and
redirects the terminal input and output to a file.

! + config
!   + policy | label_prefix: client1 | filename: test.txt
!   + policy | label_prefix: client2 | filename: file.dat | io_buffer_size: 4K
!   + libc | stdout: /dev/log
!   + vfs
!     + dir dev | + device | + log
!     + fs

To keep things simple, a client can only open one file at the moment.
