The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate dependency on io_thread_get().

The Connect method can be called between Schedule and lock. In that case, when
locked, the state is already set to CONNECTING of READY and the condition won't
be signaled anymore.

Replaces StatFile(), with a portable data object.

Return false on error, not nullptr.

Fixes bogus assertion failure.

Share the NFS connection with the NFS input plugin.

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.

"override" implies "virtual".