Implement fn close_range for MacOS

[squell]

This function exists on Linux (since we updated to a newer libc) and FreeBSD. Maybe it's as easy as finding the right IOCTL, or maybe we need to do this the slow painful way.

[bjorn3]

Looks like on macOS og-sudo sets the CLOEXEC flag for all files in the range 0..sysconf(_SC_OPEN_MAX) manually rather than closing them to prevent a libdispatch crash.

https://github.com/sudo-project/sudo/blob/116115229a625afe068ac6c593a5ce2176a96a08/lib/util/closefrom.c#L70-L76