This change adds an explicit dependency on generator.ml for every file it
generates, except java files. Java is left for another time because it's
considerably trickier.

It also adds a build rule for src/libguestfs.la so it can be rebuilt as required
from other directories.

It does this by creating a top level make file, subdir-rules.mk, which can be
included from sub-directories. sub-directories need to define 'generator_built'
to include local files which are built by generator.ml, and they will be updated
automatically.

This fixes parallel make, and will automatically re-create generated files when
make is run from any directory.

It also fixes the problem which efad4f53 was targetting. Specifically,
src/guestfs_protocol.(c|h) had an erroneous dependency on stamp-generator, and
therefore generator.ml, despite not being directly created by it. This caused
them to be recreated every time generator.ml ran rather than only when
src/guestfs_protocol.x was updated, which cascaded into a daemon and therefore
appliance update.

This patch also changes the contents of the distribution tarball by including
files created by rpcgen.

We have to include this generated file because it is part of
the build system, thus required to exist before the generator
runs.

This patch allows you to do:
mkdir build
cd build
../configure ...
make

This will output all generated files to the build directory. Given that
autogen automatically runs configure, you can also do:

BUILDDIR=./build ./autogen.sh

which will do the right thing.

Also:

* Fix a dependency bug which means that guestfs_protocol.h
  isn't automatically rebuilt.
* Re-running autogen.sh with no arguments won't blow away your previous
  configure arguments.

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.