`{host,target}-os` configuration keys semantics

[dbuenzli]

Give a meaningful and accurate description of possible values and have a sound value discovery logic.

Here's what happens now for host-os: if Sys.os_type is not Win32, it's uname -s, otherwise it is Win32. For target-os it's the value of host-os if unspecified on the command line.

Now do we get a meaningful value on cygwin ? Should we treat cygwin as a unix ? Should we also have a derived simpler value with a finite number of cases (like Sys.os_type) ?

[dbuenzli]

See also ocaml-deptext's corresponding functionality.