Add a runtime sanity check to avoid the assertion failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

This attribute is not a URI; it is just the filename without its
parent directory path.  To avoid confusion, let's rename it to
"filename", leaving the struct without a "uri" attribute.

No longer allocate it as a "VarSize".  This used to be a clever trick
to save memory 10 years ago, but these days, keeping the code
maintainable seems more important than saving a few kilobytes of
memory.

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.

Supports .mpdignore on NFS/SMB and others (closes #290).

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

Closes #169

Signed-off-by: FlashSystems <developer@flashsystems.de>

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

Wildcard matches are directly applied to all filenames in
subdirectories without any attempt at matching relative paths.

This change is based on the following feature request:

  http://bugs.musicpd.org/view.php?id=3729

std::auto_ptr is deprecated, and std::unique_ptr is much better
anyway.