Was using the wrong parameter.

Fixes
../src/time/ISO8601.cxx:67:24: error: use of undeclared identifier 'strtoul'
        unsigned long value = strtoul(s, &endptr, 10);
                              ^
../src/time/ISO8601.cxx:77:14: error: use of undeclared identifier 'strtoul'
                        minutes = strtoul(s, &endptr, 10);
                                  ^

on NetBSD with clang 9.0.0.

Caused by 2bc127bb

This prepares the migration away from strptime() for Windows
portability.

But the real reason I'm doing this is that strptime() on Apple is
buggy: strptime("14", "%H%M%S") (without separating colons) succeeds
even though only the hour has been parsed.  This fixes recent Travis
failures in the ParseISO8601() unit test.

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

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