[patch] add "underwave" face attribute

[Aurélien Aptel]

Hi all,

I implemented a new face attribute "underwave". It does exactly what
this thread[1] asked for.
It only works on X11 currently. I don't have a windows/mac machine to
port this feature but it should be easy to do: just port the changes
made to xterm.c to other *term.c. I've basically followed whatever
"overline" was doing so it should be ok.

I've attached the diff and a screenshot of the splash screen with the
default and link face changed to use underwaving. Note that it looks
good on both X11 core fonts and Xft fonts.

I would love to this this merged in emacs so -- if everything's
alright -- what's the next step?

1: http://thread.gmane.org/gmane.emacs.devel/147873/focus=147905

underwave.diff
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sample.png
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