synchronized with lua-nginx-module #aef03eb, #ee47e55, #afeeb84, and #f87d1f1.

feature: added the C API to support the 'ngx.pipe' module provided by
lua-resty-core.

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bugfix: ngx.pipe: proc:wait() returns false only when the sub-process
exited abnormally.

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bugfix: ngx.pipe: fixed a segfault when error_log is configured with
syslog.

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bugfix: ngx.pipe: cleared the pending event after aborting the uthread.

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Thanks Zexuan Luo for the upstream patches.

Author: thibaultcha