Commit the userland portion of the posix_spawn_chdir project by Piyush Sachdeva

Remove paranthesis from return operands.

Limit the amount of kernel memory a posix_spawn syscall can use (for handling
the file action list) by limiting the maximum number of file actions to
twice the current file descriptor limit.
Fix a fe

Limit the amount of kernel memory a posix_spawn syscall can use (for handling
the file action list) by limiting the maximum number of file actions to
twice the current file descriptor limit.
Fix a few bugs in the support functions and document the new limit.
From Maxime Villard.

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Rework posix_spawn locking and memory management:
- always provide a vmspace for the new proc, initially borrowing from proc0
(this part fixes PR 46286)
- increase parallelism between parent and

Rework posix_spawn locking and memory management:
- always provide a vmspace for the new proc, initially borrowing from proc0
(this part fixes PR 46286)
- increase parallelism between parent and child if arguments allow this,
avoiding a potential deadlock on exec_lock
- add a new flag for userland to request old (lockstepped) behaviour for
better error reporting
- adapt test cases to the previous two and add a new variant to test the
diagnostics flag
- fix a few memory (and lock) leaks
- provide netbsd32 compat

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Add userland part of posix_spawn. Libc functions imported from FreeBSD.
Based on Charles Zhang's summer of code project.