This makes xsetenv internal to x.c, and allows iso14755's external
command to use $WINDOWID instead of having to snprintf it again.  (The
same benefit will apply to the externalpipe patch.)  The xwinid function
is no longer needed.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

The alpha value needs to be initialized as well.

An example where the new behaviour makes more sense:

Suppose some text is formatted with ATTR_FAINT for red for the foreground, so it
is rendered in a dark red. In that case, when selected with the mouse, the
intended behaviour is that foreground and background color are swapped: so the
selection should be rendered in dark red and the text in the default background
color.

Before this patch, what happened was that the selection would be in normal red
and the text in the darkened background color, making it almost unreadable.

For an example application that uses the FAINT attribute, try dmesg from
util-linux with color support, it uses FAINT for segfault messages.

This reverts commit 77c51c5a6b.

Having multiple clipboards are useful, for example for plumber scripts.
I've discussed this on IRC and it is useful to have.

Thanks to tarug0 for the suggestion/patch.

st currently does not keep any mode for the cursor that was active
in the underlying glyph (e.g. italic text), the mode is always
ATTR_NULL [1].  At [2] you can find a screenshot that shows the
implications.  Other terminals (at least vte-based, such as
XFCE-terminal) keep some modes for the cursor.  I find the current
behaviour very disruptive, so here is a patch that keeps a few
(arbitrarily chosen) modes for the cursor.

[1] http://git.suckless.org/st/tree/st.c#n3963
[2] http://i.imgur.com/R2yCEaC.png

This is used by, e.g., tmux.