Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

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

Closes #169

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

Don't define HAVE_FS_CHARSET, and make GetFSCharset() return "ACP"
instead of "UTF-8".  Ignore the configuration setting, which had no
effect anyway.

Use wchar_t for everything on Windows.  Solves a lot of filesystem
charset problems.

New in clang 3.6.

Prepare for a ICU-based backend without GLib.

Don't expose pointer that requires the caller to invoke g_free(),
because that's GLib-only.

Don't abort the process, let the caller decide instead.

MPD_PATH_MAX_UTF8 is only used by GLib-specific code currently.

The upper-case name seems to be the canonical one.

If fs_charset is empty, i.e. we're using the default "utf-8",
GetFSCharset() should return exactly that instead of an empty
std::string.

Implement a fast path for UTF-8 which leaves fs_charset empty, and
don't assign a value to fs_charset if there's no configuration.