This is the per-I/O equivalent of O_APPEND to support atomic append
operations on any open file.

If a file is opened with O_APPEND, pwrite() ignores the offset and
always appends data to the end of the file. RWF_APPEND enables atomic
append and pwrite() with offset on a single file descriptor.
Signed-off-by: NJürg Billeter <j@bitron.ch>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>