external special remotes mostly implemented (untested)

This has not been tested at all. It compiles!

The only known missing things are support for encryption, and for get/set
of special remote configuration, and of key state. (The latter needs
separate work to add a new per-key log file to store that state.)

Only thing I don't much like is that initremote needs to be passed both
type=external and externaltype=foo. It would be better to have just
type=foo

Most of this is quite straightforward code, that largely wrote itself given
the types. The only tricky parts were:

* Need to lock the remote when using it to eg make a request, because
  in theory git-annex could have multiple threads that each try to use
  a remote at the same time. I don't think that git-annex ever does
  that currently, but better safe than sorry.

* Rather than starting up every external special remote program when
  git-annex starts, they are started only on demand, when first used.
  This will avoid slowdown, especially when running fast git-annex query
  commands. Once started, they keep running until git-annex stops, currently,
  which may not be ideal, but it's hard to know a better time to stop them.

* Bit of a chicken and egg problem with caching the cost of the remote,
  because setting annex-cost in the git config needs the remote to already
  be set up. Managed to finesse that.

This commit was sponsored by Lukas Anzinger.

Types/GitConfig.hs

@@ -113,6 +113,7 @@ data RemoteGitConfig = RemoteGitConfig
 	, remoteAnnexDirectory :: Maybe FilePath
 	, remoteAnnexGCrypt :: Maybe String
 	, remoteAnnexHookType :: Maybe String
+	, remoteAnnexExternalType :: Maybe String
 	{- A regular git remote's git repository config. -}
 	, remoteGitConfig :: Maybe GitConfig
 	}
@@ -137,6 +138,7 @@ extractRemoteGitConfig r remotename = RemoteGitConfig
 	, remoteAnnexDirectory = notempty $ getmaybe "directory"
 	, remoteAnnexGCrypt = notempty $ getmaybe "gcrypt"
 	, remoteAnnexHookType = notempty $ getmaybe "hooktype"
+	, remoteAnnexExternalType = notempty $ getmaybe "externaltype"
 	, remoteGitConfig = Nothing
 	}
   where