Append an 'nexpect' command to the script. The 'nexpect' command
is used to append additional characters to the 'bexpect' string
when the desired 'bexpect' string is more than 255 characters. The
set value of this object should be the exact characters to be
appended. The 'nexpect ' characters will be automatically appended.
Also a 'null terminator'(\0) will be automatically appended.
The null terminator and the 'nexpect ' characters will be counted
as characters in the script.