ext/pcntl: Refactor `pcntl_exec()` (#17172)

* Refactor usage of strlcpy

As we allocate the buffer, we know the string will fit inside the buffer.

* Throw ValueErrors when s

ext/pcntl: Refactor `pcntl_exec()` (#17172)

* Refactor usage of strlcpy

As we allocate the buffer, we know the string will fit inside the buffer.

* Throw ValueErrors when strings contain null bytes

The underlying C calls work with C strings, which are NULL terminated.

* exec_pcntl() always return false

Thus, update stubs to formally have a return type of `false`.

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