The created window in xattach is missing the StructureNotifyMask,
if the WM does resize the window before mapping (like tiling WMs
do), the client will not get notified (and XGetWindowAttributes
in the same function will not get updated values)

Reproducible by starting acme in e.g. dwm.

x11-screen.c accepts selection targets with the MIME type
"text/plain;charset-UTF8", but it should do so case insensitively. The
current code draws an error when working with xfce4-terminal 1.0.4,
which sends lower case.  To reproduce, run acme, snarf some text, then
paste in the terminal.  acme will report:
acme: cannot handle selection request for 'text/plain;charset=utf-8' (590)

Because of a dangling else, after adding a name to an image with 'N',
it was immediately deleted, resulting in Enoname in response to 'n'.

This went mostly unnoticed, since plan9port itself does not use named
images, but it can break external applications like truedraw[1].

[1]: https://git.sr.ht/~arusekk/truedraw

Fixes: c66b52501b63 ("new draw server")

Introduce a new variable set in $PLAN9/config:
$egrep, which is conditionally set to either
`egrep` or `grep -E`, based on platform, and use
this when we want to invoke `egrep`.

This works around the endless warnings from GNU
grep stating that `egrep` is deprecated in favor
of `grep -E`, when run on e.g. Linux.

This bug is an intermittent annoyance and does not seem likely to be
fixed soon. It means that every so often my acme becomes unusable
because it behaves as if a mouse button is permanently set.

Avoid that issue by keeping track of mouse button state directly.

While we're about it, fix another annoying issue: under Linux the
FocusOut event reset the kstate field but not the kbuttons field,
meaning that the buttons state could inappropriately persist. This
happened occasionally by accident, but was easy to reproduce: with the
mouse button held down, drag outside the window and click Alt-Tab to
switch away from the window. We see a KeyDown event followed by FocusOut
but no KeyUp event.

Change-Id: I3708316478c80c41806d7157ae9929c6cd84b662

An experiment. Let's see if it's any good.
Also document the Mac conventions in devdraw(3).