Fixes buffer overflow bug.

MPD handles all strings in UTF-8 internally.  Those decoders which
read Latin-1 tags are supposed to implement the conversion, instead of
passing Latin-1 to TagBuilder::AddItem().  FixTagString() is simply
the wrong place to do that, and hard-coding Latin-1 is kind of
arbitrary.

Share the NFS connection with the NFS input plugin.

Auto-stop the IO thread in all error handlers.

Read all directory entries into memory and close the struct nfsdir
before returning the StorageDirectoryReader instance.  This is what
libnfs does, anyway.

Creating a NfsStorage sets its own export_name as the "base".  Now
NfsFileReader can use this information to derive the export_name to be
mounted, instead of guessing.  This solves the "too many connection"
problem on the NFS server while updating the database.

Convert filesystem charset to UTF-8 for playlist_list_open_uri().
This fixes one of many remaining charset bugs.

Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

Combines the "file" and the "archive" input plugins.

This special case was useless.  Fixes the dreaded "Unrecognized URI"
error message when a file does not exist.