Fix ignored Motif scrollbar resources (Bug#37359)

Fix ignored Motif scrollbar resources (Bug#37359)

* src/xterm.c (x_create_toolkit_scroll_bar): On Motif
scrollbars, "foreground" has no meaning, while "background"
means the truck and arrow colors, and "trough" means the
background of the entire widget.  This fix hooks up the Emacs
scrollbar "foreground" color to the XmNbackground resource
and the "background" color to XmNtroughColor which is more in
line with how Xaw scrollbars behave.

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