Bind the easy parts of the 'btrfs' program.

The new APIs are:

btrfs-device-add: add devices to a btrfs filesystem
btrfs-device-delete: remove devices from a btrfs filesystem
btrfs-filesystem-sync: sync a btrfs filesystem
btrfs-filesystem-balance: balance a btrfs filesystem
btrfs-subvolume-create: create a btrfs snapshot
btrfs-subvolume-delete: delete a btrfs snapshot
btrfs-subvolume-list: list btrfs snapshots and subvolumes
btrfs-subvolume-set-default: set default btrfs subvolume
btrfs-subvolume-snapshot: create a writable btrfs snapshot

This returns information about the underlying devices of an MD
(software RAID) device.

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 : $_'

This modifies the way that struct and struct lists are generated
(for return values) so that there is no need to add an explicit
new type when adding a new structure.

All tests pass, and the C API should be compatible.

I have also inspected the changes that are made to the generated
code by hand.

Git users now require the OCaml compiler in order to regenerate
the generated code after a checkout.

Generated code will remain in the tarball, so users of the
source tarball will not need the OCaml compiler.

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.