plan9port/bin/disk/.placeholder, branch patch

plan9port/bin/disk/.placeholder, branch patch Plan 9 from User Space (with patches) disk/mkfs, disk/mkext: add from Plan 9 2012-07-17T23:10:45+00:00 Russ Cox rsc@swtch.com 2012-07-17T23:10:45+00:00 d2173bb552d308d60a4e4a53cd3b8e0949b38dbc R=rsc, rsc http://codereview.appspot.com/6405057

R=rsc, rsc
http://codereview.appspot.com/6405057