softmmu/physmem: Distinguish between file access mode and mmap protection

There is a difference between how we open a file and how we mmap it,
and we want to support writable private mappings of readonly files. Let's
define RAM_READONLY and RAM_READONLY_FD flags, to replace the single
"readonly" parameter for file-related functions.

In memory_region_init_ram_from_fd() and memory_region_init_ram_from_file(),
initialize mr->readonly based on the new RAM_READONLY flag.

While at it, add some RAM_* flags we missed to add to the list of accepted
flags in the documentation of some functions.

No change in functionality intended. We'll make use of both flags next
and start setting them independently for memory-backend-file.

Message-ID: <20230906120503.359863-3-david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>