Add support for gdb.PYTHONDIR as $gdb_datadir/python.

Add support for  gdb.PYTHONDIR as $gdb_datadir/python.

Python scripts to be used by GDB can be stored in that directory,
and will be automatically found by the interpreter when importing them.
This patch also sets up <gdb_pythondir>/gdb as the directory where gdb
submodules can be stored.  For now, there is nothing there, but it can
now be added easily without further code changes.

gdb/ChangeLog:
2010-06-24  Joel Brobecker  <brobecker@adacore.com>

        * python/python.c (_initialize_python): Add new "constant"
        PYTHONDIR in gdb module.  Insert this path at the head of
        sys.path. Set gdb.__path__ to gdb.PYTHONDIR + '/gdb' and
        exec its __init__.py script if it exists in that directory.