convert all bytestring readFile/writeFile to use Utility.FileIO

convert all bytestring readFile/writeFile to use Utility.FileIO

This is groundwork for setting the close-on-exec flag when opening
files, which will be done in Utility.FileIO or a similar module using the same
function names. The bytestring library does not set that flag, even though it
could, and IMHO should.

Note that there are many calls to the Prelude's readFile/writeFile/appendFile
still in git-annex, and this does not address those.

Sponsored-by: the NIH-funded NICEMAN (ReproNim TR&D3) project