set up the display for testing.
(gdbtk_start): Convert all paths to paths that tcl will like.
Export target information to environment.
(_gdbtk_xvfb_init): New proc to start Xvfb if available and
necessary.
(_gdbtk_xvfb_exit): New proc to kill Xvfb if necessary.
(to_tcl_path): New proc to convert a given pathname into
a path acceptible as an argument to a tcl command.
(_gdbtk_export_target_info): New proc to export target info
into the environment for gdbtk testing.
(gdbtk_done): New proc to signal end-of-test.
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