fcntl on a guest socket handled F_GETFL and F_SETFL only and failed every other command with EINVAL, so libcurl's fcntl(fd, F_SETFD, FD_CLOEXEC) on a new socket failed (ProsperoRadio PPSA99001, traced by screamm on #4796). Record FD_CLOEXEC per socket descriptor: F_SETFD sets or clears it, F_GETFD reads it, SOCK_CLOEXEC sets it at creation, and dup, dup2 and close start without it, as FreeBSD's kern_fcntl and kern_dup do.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
