release v0.21.17

Closes https://github.com/MusicPlayerDaemon/MPD/issues/685

Prepare for allowing ISO8601 time stamps without delimiters, such as
20191216, and prevent them from being interpreted as numeric UNIX time
stamps.

And allow "Z" suffix after date.

Prepare for adding more flexible parsing.

Move code from Parser.cxx.

This is recommended by the strptime() manpage, because strptime() does
not initialize/set attributes which were not specified in the format
string.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/562

Most importantly, this commit translates ZZIP_ENOENT to
std::system_error(ENOENT) so IsFileNotFound() returns true and
find_stream_art() can suppress the log line.