acme: add log file in acme root directory

Reading /mnt/acme/log reports a log of window create,
put, and delete events, as they happen. It blocks until the
next event is available.

Example log output:

8 new /Users/rsc/foo.go
8 put /Users/rsc/foo.go
8 del /Users/rsc/foo.go

This lets acme-aware programs react to file writes, for example
compiling code, running a test, or updating an import block.

TBR=r
R=r
https://codereview.appspot.com/89560044

src/cmd/acme/util.c

@@ -97,6 +97,7 @@ errorwin1(Rune *dir, int ndir, Rune **incl, int nincl)
 		w = coladd(row.col[row.ncol-1], nil, nil, -1);
 		w->filemenu = FALSE;
 		winsetname(w, r, n);
+		xfidlog(w, "new");
 	}
 	free(r);
 	for(i=nincl; --i>=0; ){