proposal: x/exp/mmap: support madvise syscall

[samueleresca]

Proposal Details

Proposal

This proposal is to add support of the madvise(2) syscall to the golang.org/x/exp/mmap package. Specifying madvise can help to guide the kernel on how to handle the mmap'd file.
To support madvise, it is necessary to:

• Expose the call through the syscall package
• Expose configurations in the golang.org/x/exp/mmap package will be specified at mmap.Open

In addition to the above, it would be great to be able to specify a flag different from syscall.MAP_SHARED for the mmap'd file. See current implementation.

These new configurations can then be chosen using an option struct to pass to the mmap.Open call.