Make the File.open constructor use read-only mode (Issue #2567)

[tmmcguire]

Use @ponyint_o_rdonly rather than _rdwr in the calls to @OPEN and
@_open.

[slfritchie]

I hadn't paid much attention to File's API until this ticket. It seems odd to me (wearing a C programmer's hat) that so little control is available to a Pony programmer for open(2)'s 2nd and 3rd arguments ... but that's a matter for a different PR or perhaps even RFC.

In this case, it's pretty clear to me that this PR fixes a bug. The docstring for File.open says "Open for read only" and also desires read-only behavior from the rest of the class by using writable = false. � Anybody have a different opinion?

[jemc]

I agree. Should we go ahead and merge this?

[jemc]

Discussed on sync call.

We can merge this as-is, because it fixes an issue and doesn't make anything worse.

However, we need to spend some time looking at how to handle errors where you open a file with read-only permissions and try to write to it. It appears to me that such writes are silently swallowed at runtime, rather than raising an error at compile time or runtime. @mfelsche is opening an issue to discuss this.