Breaking: Remove openmode parameter from file construction

[bugdea1er]

According to C99 Rationale, unnamed temporary files are expected to be written first and then read as transparently as possible, so they are created in binary update mode

7.19.4.3 The tmpfile function

The tmpfile function is intended to allow users to create binary “scratch” files. The as if principle implies that the information in such a file need never actually be stored on a file-structured device.
The temporary file is created in binary update mode because it will presumably be first written and then read as transparently as possible. Trailing null-character padding may cause problems for some existing programs.

https://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf

I don't see any point in implementing tmp::file in any other way, given that we haven't found a practical use for text or append modes in production for over a year now