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.

Command/ImportFeed.hs

@@ -40,6 +40,7 @@ import Logs.MetaData
 import Annex.MetaData
 import Annex.FileMatcher
 import Command.AddUrl (addWorkTree)
+import Annex.UntrustedFilePath
 
 cmd :: Command
 cmd = notBareRepo $