Make -b output host file length as close as PDP-10 length as possible.

[larsbrinkhoff]

In particular, for 7-bit files, the output file will have the same number of bytes as the PDP-10 file. For 36-bit files, the output file size will always be a multiple of five as before. For other byte sizes, the output file will have the minimum number of octets needed to represent the PDP-10 data using the "swizzled" format.

CC @bictorv