ends up pointing to the wrong place before it is passed to the client.
The path is only used internally so there is no real need for
realpath(), remove it and move the get_path function to file.c where all
the callers are.
not attached, the server process asks it to open the file, similar to
how works for stdin, stdout, stderr. This makes special files like
/dev/fd/X work (used by some shells). stdin, stdout and stderr and
control mode are now just special cases of the same mechanism. This will
also make it easier to use for other commands that read files such as
source-file.
