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/fsys.c

@@ -71,6 +71,7 @@ Dirtab dirtab[]=
 	{ "editout",	QTFILE,	Qeditout,	0200 },
 	{ "index",		QTFILE,	Qindex,	0400 },
 	{ "label",		QTFILE,	Qlabel,	0600 },
+	{ "log",		QTFILE,	Qlog,	0400 },
 	{ "new",		QTDIR,	Qnew,	0500|DMDIR },
 	{ nil, }
 };