Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

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.

Unused.

Referencing the attribute "partition" is illegal after Close(),
because Close() deletes "this".

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().

Replaces StatFile(), with a portable data object.

Eliminate extra allocations for the std::list node instances.

Use Storage::MapUTF8() internally, don't use global variables.

Replaces global variable "main_loop".