fork: update the thread id in the pthread struct after forking

Otherwise the child process's main thread will inherit the thread id
of its parent and pthread operations will be directed at the wrong
thread.

src/system/libroot/posix/unistd/fork.c

@@ -13,6 +13,7 @@
 #include <errno_private.h>
 #include <locks.h>
 #include <libroot_private.h>
+#include <pthread_private.h>
 #include <runtime_loader.h>
 #include <syscalls.h>
 
@@ -147,6 +148,8 @@ fork(void)
 		// we are the child
 		// ToDo: initialize child
 		__main_thread_id = find_thread(NULL);
+		pthread_self()->id = __main_thread_id;
+
 		mutex_init(&sForkLock, FORK_LOCK_NAME);
 			// TODO: The lock is already initialized and we in the fork()ing
 			// process we should make sure that it is in a consistent state when