Add inherit fd support

There are cases where a process's file descriptor cannot be restored
from the checkpoint images.  For example, a pipe file descriptor with
one end in the checkpointed process and the other end in a separate
process (that was not part of the checkpointed process tree) cannot be
restored because after checkpoint the pipe will be broken.

There are also cases where the user wants to use a new file during
restore instead of the original file at checkpoint time.  For example,
the user wants to change the log file of a process from /path/to/oldlog
to /path/to/newlog.

In these cases, criu's caller should set up a new file descriptor to be
inherited by the restored process and specify the file descriptor with the
--inherit-fd command line option.  The argument of --inherit-fd has the
format fd[%d]:%s, where %d tells criu which of its own file descriptors
to use for restoring the file identified by %s.

As a debugging aid, if the argument has the format debug[%d]:%s, it tells
criu to write out the string after colon to the file descriptor %d.  This
can be used, for example, as an easy way to leave a "restore marker"
in the output stream of the process.

It's important to note that inherit fd support breaks applications
that depend on the state of the file descriptor being inherited.  So,
consider inherit fd only for specific use cases that you know for sure
won't break the application.

For examples please visit http://criu.org/Category:HOWTO.

v2: Added a check in send_fd_to_self() to avoid closing an inherit fd.
    Also, as an extra measure of caution, added checks in the inherit fd
    look up functions to make sure that the inherit fd hasn't been reused.
    The patch also includes minor cosmetic changes.

Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>