#23214 has a few glitches, this is a PR fixing one of them :)

there were 3 issues with File as pointed out by #23436:

File.open

in the spec, the OpenMode argument can only be read, read|write, or read|write|execute. However, currently the interface only allows read, write, and execute. This PR addresses this problem:

• defined an auxiliary type OpenMode.Bits which provides a bitfield representing those values from the spec
• OpenMode's enum tags are renamed to read, read_write, and read_write_create for better clarity
• OpenMode's enum values are implemented by bitcasting OpenMode.Bits values

File.getInfo

The file info buffer should be larger than @sizeOf(Info.*), because Info.* types have an attached null-terminated string at the end.

This method now requires the user to pass in a buffer which will contain the resulting info type.

To better align with the pattern being established in #23441, this function's return type is now struct { usize, ?*Payload }.

File.setInfo

The len accounting as-is is wrong. It should account for the length of the attached null-terminated string.

closes #23436