cothread_create/cothread_delete ignore pthread_create()/pthread_join() failure — EAGAIN leaves a registered virtual interrupt, a NULL pthread handle, and undefined pthread_kill/pthread_join on it

Summary

pthread_create's return is unchecked: on EAGAIN the M_ZERO'd cotd->pthr stays NULL while thr_intr remains registered on virtual IRQ 1 and the caller believes it has a working cothread - every later cothread_intr does pthread_kill(NULL) and delete does pthread_join(NULL) (undefined per libthread_xu, likely host-process crash), and the cotd leaks. Host-level thread exhaustion (RLIMIT_NPROC pressure from an unprivileged user on a shared host) converts a recoverable EAGAIN into a vkernel crash or silently dead device channel. check+unwind fix.