syscall: call tcsetpgrp from parent thread when spawning a foreground child process

[pupper68k]

Currently we call tcsetpgrp from child thread causing the foreground process to hang.
The child pgrp is either found in the SysProcAttr or equal to the child pid.
Since we have child pgrp we can call tcsetpgrp from the parent thread.
Fixes #37217

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[pupper68k]

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(1 comment)

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[gopherbot]

This PR is being closed because golang.org/cl/266057 has been abandoned.

Let's use CL 313653 instead.