add a init program

[xudifsd]

Some benefit of init program:

• reap zombie processes
• capture signal and print out for debug
• capture exit signal and broadcast to user's process, allow graceful exit, also improve exit speed

compiled init program has 2.2MB, and takes 5s to terminate the pod instead of 30s.

Before this, integration test takes 3m56s:

real    3m56.608s
user    0m11.417s
sys     0m2.814s

After this, takes 2m35s:

real    2m35.153s
user    0m10.407s
sys     0m2.847s

[coveralls]

Pull Request Test Coverage Report for Build 3497

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.41%

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Totals
Change from base Build 3495:	0.0%
Covered Lines:	816
Relevant Lines:	863

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💛 - Coveralls