LE modification for fork thread behaviour

There are multiple instances in the the core python implementation and in pypi packages where a thread calls fork and the child process tries to create a thread using pthread_create but results in an error. 
This error is due to the LE design implementation and relates to the error code:
ELEMULTITHREADFORK - pthread_create() was invoked from a child process created by calling fork() from a multi-threaded process. This child process is restricted from becoming multi-threaded.


A short code example that showcases this behaviour is as follows:
```
#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/wait.h>
void * thread_func(void * unused) {
    return NULL;
}
int main() {
    int status = 0;
    pthread_t t1;
    pthread_create(&t1, NULL, thread_func, (void*)NULL);
    int pid = fork();
    if (pid == 0) {
        printf("Child process. \t\tParent_pid: %d Child_pid: %d.\n", getppid(), getpid());
        pthread_t t2;
        errno = 0;
        pthread_create(&t2, NULL, thread_func, (void*)NULL);
        exit(errno);
    } 
    else if (pid > 0) {
        printf("Parent process. \tParent_pid: %d Child_pid: %d.\n", getpid(), pid);
        waitpid(pid, &status, WUNTRACED | WCONTINUED);
        printf("status: %d\n", status);
    } else 
    {
        printf("Error creating child process.\n");
    }
    return 0;
}
```


To be able properly support community-developed code which just works on other platforms, and to lower the friction and barrier to getting new important workloads working on z/OS, we are requesting this feature from LE, that allows the creation of threads invoked from a child process created by calling fork() from a multi-threaded process.


Reference
https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.4.0/com.ibm.zos.v2r4.bpxbd00/ptcrea.htm