file: add a general approach for inherited files

How to restore an external file? The problem is that a file
can be opened with different flags (O_APPNED, O_RDWR, O_RONLY,
etc). These flags can't be changed for a file descriptor, so
we can't just duplicate an inherit fd, when we meet an external
file.

A file is external, when we can't access them directly.
CRIU is able to restore a file descriptor, if it knows
how to open it, so we need to provide a way to open
an external file.

The idea of this patch is that we can open an external file
via /proc/self/fd/X where X is an inherit fd.

This approach works for files and fifo.

An example how it can be used:
criu -o dump.log -D dump/fifo.py/6/1 -v4 -t 6 --external file[72:a3e7]
criu -o restore.log -D dump/fifo.py/6/1 -v4 --restore-sibling --inherit-fd fd[4]:file[72:a3e7] --restore-detached

https://github.com/xemul/criu/issues/104



Cc: Saied Kazemi <saied@google.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>