The ALSA "null" driver opens /dev/null and returns the file handle
from snd_pcm_poll_descriptors(), but /dev/null cannot be used with
epoll, the epoll_ctl() system call returns -EPERM.  This means that
the ALSA output hangs, eventually freezing the whole MPD process.

This commit adds a workaround to the MultiSocketMonitor class which is
used by the ALSA output plugin.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/695

SetReady() does this already.

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.