added preferred-content log, and allow editing it with vicfg

This includes a full parser for the boolean expressions in the log,
that compiles them into Matchers. Those matchers are not used yet.

A complication is that matching against an expression should never
crash git-annex with an error. Instead, vicfg checks that the expressions
parse. If a bad expression (or an expression understood by some future
git-annex version) gets into the log, it'll be ignored.

Most of the code in Limit couldn't fail anyway, but I did have to make
limitCopies check its parameter first, and return an error if it's bad,
rather than erroring at runtime.

Utility/Matcher.hs

@@ -19,6 +19,7 @@ module Utility.Matcher (
 	Token(..),
 	Matcher,
 	token,
+	tokens,
 	generate,
 	match,
 	matchM,
@@ -48,6 +49,9 @@ token "(" = Open
 token ")" = Close
 token t = error $ "unknown token " ++ t
 
+tokens :: [String]
+tokens = words "and or not ( )"
+
 {- Converts a list of Tokens into a Matcher. -}
 generate :: [Token op] -> Matcher op
 generate = go MAny