Apparently, invoking fcntl(fd, F_SETFL, O_DIRECT) will lead to
unexpected behaviour on Linux bcachefs and possibly other file systems,
depending on the operating system version. So, let us avoid doing that,
and instead just attempt to pass the O_DIRECT flag to open(). This should
make us compatible with NetBSD, IBM AIX, as well as Solaris and its
derivatives.

This fix does not change the fact that we had only implemented
innodb_log_file_buffering=OFF on systems where we can determine the
physical block size (typically 512 or 4096 bytes).
Currently, those operating systems are Linux and Microsoft Windows.

HAVE_FCNTL_DIRECT, os_file_set_nocache(): Remove.

OS_FILE_OVERWRITE, OS_FILE_CREATE_PATH: Remove (never used parameters).

os_file_log_buffered(), os_file_log_maybe_unbuffered(): Helper functions.

os_file_create_simple_func(): When applicable, initially attempt to
open files in O_DIRECT mode.

os_file_create_func(): When applicable, initially attempt to
open files in O_DIRECT mode.
For type==OS_LOG_FILE && create_mode != OS_FILE_CREATE
we will first invoke stat(2) on the file name to find out if the size
is compatible with O_DIRECT. If create_mode == OS_FILE_CREATE, we will
invoke fstat(2) on the created log file afterwards, and may close and
reopen the file in O_DIRECT mode if applicable.

create_temp_file(): Support O_DIRECT. This is only used if O_TMPFILE is
available and innodb_disable_sort_file_cache=ON (non-default value).
Notably, that setting never worked on Microsoft Windows.

row_merge_file_create_mode(): Split from row_merge_file_create_low().
Create a temporary file in the specified mode.

Reviewed by: Vladislav Vaintroub