* fileio.c: Use O_APPEND to append.
This corresponds better to the natural interpretation of "append", and avoids the need to open the output file twice, or to invoke lseek when APPEND is neither nil nor a number. This relies on POSIX 1003.1-1988 or later, which is OK nowadays. (Fwrite_region): Simplify. Use O_APPEND instead of opening the file possibly twice, and lseeking to its end; this avoids the need to lseek on non-regular files. Do not use O_EXCL and O_TRUNC at the same time: the combination is never needed and apparently it doesn't work with DOS_NT.
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