Set O_APPEND mode for stdout/stderr and output-sync temporary files.

Set O_APPEND mode for stdout/stderr and output-sync temporary files.

POSIX does not guarantee that writes will be atomic if a file is
opened for normal (non-append) output.  That means if multiple processes
are writing to the same file, output could be lost.  I can't think of
a real use-case where we would NOT want append for stdout/stderr, so
force it if we can.