Stdio.File: Fix grantpt() on Solaris (and others?)
grantpt(3C) on Solaris is implemented as fork + exec of
"/usr/lib/pt_chmod". Unfortunately the libc function doesn't bother
clearing the FD_CLOEXEC flag before the exec, so the fd never got
to the helper binary, which caused grantpt(3C) to fail with EPERM.