crippled filesystem support, probing and initial support

git annex init probes for crippled filesystems, and sets direct mode, as
well as `annex.crippledfilesystem`.

Avoid manipulating permissions of files on crippled filesystems.
That would likely cause an exception to be thrown.

Very basic support in Command.Add for cripped filesystems; avoids the lock
down entirely since doing it needs both permissions and hard links.
Will make this better soon.

Types/KeySource.hs

@@ -12,9 +12,12 @@ module Types.KeySource where
  -
  - The contentLocation may be different from the filename
  - associated with the key. For example, the add command
- - temporarily puts the content into a lockdown directory
+ - may temporarily hard link the content into a lockdown directory
  - for checking. The migrate command uses the content
- - of a different Key. -}
+ - of a different Key.
+ -
+ - 
+ -}
 data KeySource = KeySource
 	{ keyFilename :: FilePath
 	, contentLocation :: FilePath