Add a hook before fork() for getaddrinfo_a()

We need stop worker threads in getaddrinfo_a() before fork().
This change adds a hook before fork() that cancel all outstanding requests
and wait for all ongoing requests. Then, it waits for all worker
threads to be finished.

Fixes [Bug #17220]

include/ruby/internal/intern/process.h

@@ -28,6 +28,8 @@
 RBIMPL_SYMBOL_EXPORT_BEGIN()
 
 /* process.c */
+RUBY_EXTERN void (* rb_socket_before_exec_func)();
+
 void rb_last_status_set(int status, rb_pid_t pid);
 VALUE rb_last_status_get(void);
 int rb_proc_exec(const char*);