Add flexible buffered I/O based on fopencookie(3)

Add flexible buffered I/O based on fopencookie(3)

to be able to read and write large buffers below /proc.
The buffers and file offsets are handled dynamically
on the required buffer size at read, that is lseek(2)
is used to determine this size. Large buffers at
write are split at a delimeter into pieces and also
lseek(2) is used to write each of them.

Signed-off-by: Werner Fink <werner@suse.de>