python - be: Fix Portal seeking.

Log Message:
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Fix Portal seeking. _port_seek was mostly broken. Subsequently, seek based
operations like __getitem__ and __len__ were as well. Do error checking in
port_read to be sure the given object is a PyInt.

Additionally, add the "scroll" method. Like seek, but implies whence = 1, and
does not return the position difference.

Modified Files:
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be/src/call:
portal.c (r1.6 -> r1.7)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/call/portal.c.diff?r1=1.6&r2=1.7)