Append an 'offset' command to the script. The set value of this
object should be the offset from the beginning of the data area
of the TCP/UDP packet to start matching the string configured
in the last bexpect command. The 'offset ' characters will be
automatically appended. Also a 'null terminator'(\0) will be
automatically appended. The null terminator and the 'offset '
characters will be counted as characters in the script. When
read, the return value has no significant meaning, but a 1 is
always returned.