Pengzhen Cao noticed that read-file would fail for files
larger than the protocol size; this is *not* the bug.  However
it would also lose protocol synchronization after this.

The reason was that functions which return RBufferOut in the
generator must not 'touch' the *size_r parameter along error
return paths.

I fixed read-file and initrd-cat, and I checked that pread was
doing the right thing.

This also adds regression tests for read-file with various categories
of large file.
(cherry picked from commit 42f59b28f123f53ae038df23a9abee08e959e46b)

The RPC stubs already prefix the command name to error messages.
The daemon doesn't have to do this.  As a (small) benefit this also
makes the daemon slightly smaller.

Code in the daemon such as:

  if (argv[0] == NULL) {
    reply_with_error ("passed an empty list");
    return NULL;
  }

now results in error messages like this:

  ><fs> command ""
  libguestfs: error: command: passed an empty list

(whereas previously you would have seen ..command: command:..)

Nearly every file-related function in daemons/*.c is affected:
Remove this pair of statements from each affected do_* function:
-  NEED_ROOT (return -1);
-  ABS_PATH (dir, return -1);
and change the type of the corresponding parameter to "const char *".
* src/generator.ml: Emit NEED_ROOT just once, even when there are two or
more Pathname args.

run this command:
  git grep -l -w NEED_ROOT|xargs perl -pi -e \
    's/(NEED_ROOT) \((.*?)\)/$1 (return $2)/'

run this command:
  git grep -l -w ABS_PATH|xargs perl -pi -e \
    's/(?:ABS_PATH)( \(.*?,) (.*?)\)/ABS_PATH$1 return $2)/'

%Q => simple shell quoted string
%R => path will be prefixed by /sysroot

eg. snprintf (cmd, sizeof cmd, "cat %R", path); system (cmd);

Currently /sysroot is hard-coded throughout the daemon code.

This patch turns the path into a variable so that we can change
it in future, for example to allow standalone mode to be implemented.

This patch was tested by running all the C API tests successfully.