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/mkfile

@@ -15,6 +15,7 @@ OFILES=\
 	exec.$O\
 	file.$O\
 	fsys.$O\
+	logf.$O\
 	look.$O\
 	regx.$O\
 	rows.$O\