get_lastpostdate, get_lastpostmodified filters don't pass enough parameters

[mdawaffe]

get_lastpostdate() and get_lastpostmodified() both accept a timezone parameter and filter their respective results.

However, the filter doesn't know how the timezone parameter was specified and so can't know how to modify the return value. Though core only uses a timezone value of 'GMT', it'd still be nice to know for sure what timezone is being requested when using these filters.

Attached adds the timezone parameter to those filters.

[ryan]

(In [6301]) Post timezone to lastpost filters. Props mdawaffe. fixes #5292