fix pid leak on MacOS

[Guest0x0]

It seems that kqueue will not wait the process when it issues process event. Even though we can obtain the process exit status from kqueue, we still need to manually wait the process, otherwise its pid will not be freed. When a long-running program spawn a lot of process, this can cause an exhaustion of pid. This PR fixes the issue.

[coveralls]

Pull Request Test Coverage Report for Build 487

Details

• 12 of 18 (66.67%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 76.016%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/process/process.mbt	7	8	87.5%
src/internal/event_loop/event_loop.mbt	3	5	60.0%
src/internal/event_loop/poll.mbt	2	5	40.0%

Files with Coverage Reduction	New Missed Lines	%
src/internal/event_loop/event_loop.mbt	1	74.0%

Totals
Change from base Build 485:	-0.06%
Covered Lines:	935
Relevant Lines:	1230

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💛 - Coveralls