port atlas - remove ui_warn command from top level, add maintainer

[anddam (Andrea D'Amore)]

atlas has an ui_warn at portfile root level ​at line 245, this leads to the warning being printed whenever the portfile is read and parsed, like dependency checking for atlas' dependents or even a simple port lint atlas.

Please move into a more appropriate block, post-configure seems a good candidate.

At the same time I suggest vince is marked as maintainer since most commits in last 18 months are his.