msvc: define O_ACCMODE

This constant is not defined in MSVC's headers. In UCRT's fcntl.h, _O_RDONLY, _O_WRONLY and _O_RDWR are defined as 0, 1 and 2, respectively. Yes, that means that UCRT breaks with the tradition that O_RDWR == O_RDONLY | O_WRONLY. It is a perfectly legal way to define those constants, though, therefore we need to take care of defining O_ACCMODE accordingly. This is particularly important in order to keep our "open() can set errno to EISDIR" emulation working: it tests that (flags & O_ACCMODE) is not identical to O_RDONLY before going on to test specifically whether the file for which open() reported EACCES is, in fact, a directory. Signed-off-by: Philip Oakley <philipoakley@iee.org> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>