[gfortran,patch] Add -fmodule-private option (PR 31629)

[FX Coudert]

The attached simple patch adds a -fmodule-private option to make  
module entities private as default, i.e. when you USE a module and  
compile with this option, you cannot refer to any variable that  
wasn't explicitly declared as PUBLIC. This is a tool and help for  
some people's coding style.

It's very simple as the code was already factored nicely, so it's in  
fact a one-liner (plus option-handling stuff, documentation and  
testcase). Bootstrapped and regtested on x86_64-linux, OK to commit?

FX



:ADDPATCH fortran:
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