Adopt linux' O_APPEND behavior

Merged jD91mZM2 requested to merge jD91mZM2/redoxfs:master into master

According to man open:

The file is opened in append mode. Before each write(2), the file offset is positioned at the end of the file, as if with lseek(2). The modi‐ fication of the file offset and the write operation are performed as a single atomic step.

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