API tuning: two minor changes for usability.

[daiyip]

1. Make broadcast/execute args optional.
2. Introduce 'global.napa' to access napajs module.

[msftclas]

@daiyip,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[helloshuangzi]

            var napa = require(napaDir);

we can use "global.napa" here #Resolved

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Refers to: test/zone-test.ts:465 in 133c62f. [](commit_id = 133c62f, deletion_comment = False)

[helloshuangzi]

            var napa = require(napaDir);

the same as above #Resolved

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Refers to: test/zone-test.ts:473 in 133c62f. [](commit_id = 133c62f, deletion_comment = False)