FlatPatternMatcher can't handle optional segments

[agc93]

Since SpecParser.Parse replaces missing segments with null (which get replaced with any) there's no way to match for optional arguments. For example, this should match:

/app_2_linux.tgz

to:

new VersionSpec(2,"linux",string.Empty); // "2 [linux/unknown]"

but doesn't since SpecParser.Parse actually does a new VersionSpec(2, "linux", null);, resulting in a 2 [linux/any]