Boar should flag blobs as read-only

[GoogleCodeExporter]

To reduce the risk of accidental modification of blob data, Boar should mark  
blobs as read-only, if supported by the current file system. Adjusting the 
permission flags should be done whenever "verify" has checked the consistency 
of a blob. If there is a problem setting the flags (such as when the whole file 
system is read only), failures should just be silently ignored.

Original issue reported on code.google.com by ekb...@gmail.com on 3 Jun 2012 at 5:53