Running "go vet -shadow ./..." finds all places where a variable might
be incorrectly or unnecessarily shadowed. This fixes some of them.

Our current build tags set off the linter. We will later add in more
comprehensive build tags that will be properly formatted.

We should always log the descriptor not the entire policy structure.

Now the user is persented with help when they try to access a keyring
that isn't theirs or try to use fscrypt without a user keyring linked
into the session keyring.

Chaning the --user flag to (optionally) check for a proper keyring setup
allows us to fail early in cases where we need a working keyring.

The --user flag can now be used to have the targe user (the one whose
keyring and password will be used in fscrypt) be different than the
calling user. Very usefull for things like

	sudo fscrypt purge /media/joerichey/usb --user=joerichey

which will now have privileges to drop caches, but will properly clear
the keys from the user's keyring.