inotify: Make this optional on platforms that don't have this interface.

mknod, mkfifo etc.: Make these optional on non-Unix platforms.

readdir: If d_type field is missing on the platform, set the corresponding
field to 'u'.

stat: st_blocks and st_blksize are missing on non-Unix platforms, so
set these fields to -1 in the corresponding structures.

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)/'

Do it by running this command:
[exempted files are matched via .x-sc_TAB_in_indentation]

  git ls-files \
    | pcregrep -vf .x-sc_TAB_in_indentation \
    | xargs pcregrep -l '^ *\t' \
    | xargs perl -MText::Tabs -ni -le \
      '$m=/^( *\t[ \t]*)(.*)/; print $m ? expand($1) . $2 : $_'

* daemon/readdir.c (do_readdir): Free both p and v.name, in case
only one of the allocations failed.

This adds a readdir call (mostly intended for programs).  The
return value is a list of guestfs_dirent structures.

This adds the new types 'struct guestfs_dirent' and
'struct guestfs_dirent_list', along with all the code to
return these in the different language bindings.

Also includes additional tests for OCaml and Perl bindings
to test this.