Replaces the static Duplicate() method.

To make things simpler, AllocatedString is now a non-template class.

Replaces the static Duplicate() method.

To make things simpler, AllocatedString is now a non-template class.

Using CompareStringEx() and FindNLSStringEx().

Implements a missing piece for
https://github.com/MusicPlayerDaemon/MPD/issues/820

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.

Our IcuCaseFold() fallback using strxfrm() is not actually case
insensitive.  This commit fixes the problem by switching to
strcasecmp().  That function is not guaranteed to support UTF-8, but
it's the best we can do in this sparse situation.

Closes #111

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The soure file client.c has nearly 1000 lines, time for splitting it
into smaller pieces to improve readability.

Prepare splitting client.c into several sources.

Make the client list management a separate sub-library.

The client code uses portable GLib I/O functions and doesn't need the
OS specific socket headers.

Several functions work with the wrong return type, this patch fixes
them.

When client_defer_output() aborts the connection to the client,
client_write_output() called client_write_deferred() anyway.  This
caused an assertion failure.  Fix it by checking for the "expired"
flag again after client_defer_output() returns.

I'm hunting down a bug where client->channel==NULL during I/O
operations.  These new assertions help avoid this kind of bug in the
future.