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)/'

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.