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page: fix deadlock with addpage (thanks BurnZeZ for the stacktraces)

addpage() should not be called with the display locked as it
calls showpage1() which sleeps when there are too many
processes active.

the bug was triggered by plumbing to trigger the addpage().

--- a/sys/src/cmd/page.c

+++ b/sys/src/cmd/page.c

@@ -1681,7 +1681,6 @@

 	ground = allocimage(display, Rect(0,0,1,1), screen->chan, 1, 0x777777FF);

 	display->locking = 1;

 	unlockdisplay(display);

-	drawlock(1);

 	einit(Ekeyboard|Emouse);

 	eplumb(Eplumb, "image");

@@ -1698,6 +1697,7 @@

 	for(; *argv; argv++)

 		addpage(root, *argv, popenfile, strdup(*argv), -1);

+	drawlock(1);

 	for(;;){

 		drawlock(0);

 		i=event(&e);

@@ -1802,7 +1802,9 @@

 				j = trywalk(s, plumblookup(pm->attr, "addr"));

 				if(j == nil){

 					current = root;

+					drawlock(0);

 					j = addpage(root, s, popenfile, s, fd);

+					drawlock(1);

 				}

 				showpage(j);

 			}