DateTimeZoneProviders.Bcl.GetSystemDefault() is broken on non-Windows Mono

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All versions of Mono on non-Windows platforms are broken with respect to 
DateTimeZoneProviders.Bcl.GetSystemDefault(), with the possible exception of 
MonoDroid, which I can't check.

Because:
1. TimeZoneInfo.Local always provides a TimeZoneInfo with a fixed ID of "Local".
2. TimeZoneInfo.GetSystemTimeZones() does not actually contain a TimeZoneInfo 
with ID "Local", and so calls to DateTimeZoneProviders.Bcl.GetSystemDefault() 
will fail with e.g. "DateTimeZoneNotFoundException: Time zone Local is unknown 
to source TimeZoneInfo: 3.5.0.0".

We could fix this by special-casing the local timezone in 
BclDateTimeZoneSource, by making GetIds() unconditionally include 
TimeZoneInfo.Local.Id.

Note that TimeZoneInfo.FindSystemTimeZoneById(TimeZoneInfo.Local.Id) _does_ 
work, so I believe that would be all that's needed.

[Note also that DateTimeZoneProviders.Tzdb.GetSystemDefault() will never work 
on non-Windows Mono, as we have no CLDR mapping for a Windows time zone named 
"Local".]

I'd like to explore taking this for 1.1.1, given that other fix in 1.1.1 is in 
the same area. Jon: what do you think?

Original issue reported on code.google.com by malcolm.rowe on 31 Jul 2013 at 4:20