Replaces the static Duplicate() method.

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

Per the manual for strptime, %F is equivalent %Y-%m-%d, so use that
directly.

ParseTimePoint.

%Z is a glibc extension to strptime, and is a no-op there, due to the
mapping between timezone names and their definition (especially when the
name comes from a different machine) being ambiguous / impossible.  Time
in HTTP headers is guaranteed to be UTC.

Passing an unknown format to strptime() implementations that don't
support it will generally cause them to return NULL, which will lead to
ParseTimePoint throwing an exception and ParseTimeStamp using an
unnecessary fallback.

Since the timezone name goes at the end of the string, we don't need to
use %Z to skip it (could be an issue in a different time stamp format),
so simply removing %Z works best.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1112

Regression by commit a22d1c88

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1097

Exmaple -> Example

If the constructor moves from an ExportedSong instance which refers to
somebody else's "Tag" instance, the newly constructed instance will
instead refer to its own empty "tag_buffer" field.  This broke
SimpleDatabase::GetSong(), i.e. all songs on the queue restored from
the state file or added using the "addid" command.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1089

Needed for `requestLegacyExternalStorage` (commit ca02fb77).

The "move" command doesn't allow open-ended ranges because they don't
make a lot of sense; moving an open-ended range is only possible if
the destination index is before the range, and in that case, the
client should be well aware how many songs there are.

Closes https://github.com/MusicPlayerDaemon/MPD/pull/1057

Catch errors like that early, before invalid ranges get passed to
internal MPD subsystems.