addurl --preserve-filename and a few related changes

* addurl --preserve-filename: New option, uses server-provided filename
  without any sanitization, but with some security checking.

  Not yet implemented for remotes other than the web.

* addurl, importfeed: Avoid adding filenames with leading '.', instead
  it will be replaced with '_'.

  This might be considered a security fix, but a CVE seems unwattanted.
  It was possible for addurl to create a dotfile, which could change
  behavior of some program. It was also possible for a web server to say
  the file name was ".git" or "foo/.git". That would not overrwrite the
  .git directory, but would cause addurl to fail; of course git won't
  add "foo/.git".

sanitizeFilePath is too opinionated to remain in Utility, so moved it.

The changes to mkSafeFilePath are because it used sanitizeFilePath.
In particular:

	isDrive will never succeed, because "c:" gets munged to "c_"
	".." gets sanitized now
	".git" gets sanitized now
	It will never be null, because sanitizeFilePath keeps the length
	the same, and splitDirectories never returns a null path.

Also, on the off chance a web server suggests a filename of "",
ignore that, rather than trying to save to such a filename, which would
fail in some way.

Utility/Path.hs

@@ -1,6 +1,6 @@
 {- path manipulation
  -
- - Copyright 2010-2014 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2020 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
@@ -24,7 +24,6 @@ module Utility.Path (
 	inPath,
 	searchPath,
 	dotfile,
-	sanitizeFilePath,
 	splitShortExtensions,
 
 	prop_upFrom_basics,
@@ -35,7 +34,6 @@ module Utility.Path (
 import System.FilePath
 import Data.List
 import Data.Maybe
-import Data.Char
 import Control.Applicative
 import Prelude
 
@@ -276,22 +274,6 @@ dotfile file
   where
 	f = takeFileName file
 
-{- Given a string that we'd like to use as the basis for FilePath, but that
- - was provided by a third party and is not to be trusted, returns the closest
- - sane FilePath.
- -
- - All spaces and punctuation and other wacky stuff are replaced
- - with '_', except for '.'
- - "../" will thus turn into ".._", which is safe.
- -}
-sanitizeFilePath :: String -> FilePath
-sanitizeFilePath = map sanitize
-  where
-	sanitize c
-		| c == '.' = c
-		| isSpace c || isPunctuation c || isSymbol c || isControl c || c == '/' = '_'
-		| otherwise = c
-
 {- Similar to splitExtensions, but knows that some things in FilePaths
  - after a dot are too long to be extensions. -}
 splitShortExtensions :: FilePath -> (FilePath, [String])