avoid overhead of fcntl when setting close-on-exec

avoid overhead of fcntl when setting close-on-exec

unix-2.8.0 adds cloexec to OpenFileFlags, rather than needing to call
setFdOption after opening.

This also might avoid a race, if another thread started a process at
just the wrong time, before the flag got set, it could inherit the FD.

Unfortunately, the arm64-ancient build needs support for older versions
of unix than that, so had to ifdef. That build is still needed to
support using git-annex in termux on phones like mine, I have confirmed
today.

Sponsored-by: Luke T. Shumaker