narrow write-seek-write's flags to only use O_RDWR | O_CREAT

It was enabling unecessary flags (e.g., O_APPEND)
Issue #250: implement O_APPEND in sqlfs_proc_write

As per man 2 open: "Before each write(2), the file offset is
positioned at the end of the file"

Also, as per man 2 write: "If the file was open(2)ed with
O_APPEND, the file offset is first set to the end of
the file before writing."
add a unit test for O_APPEND functionality