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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.