MINOR: ist: add ist0() to add a trailing zero to a string.

MINOR: ist: add ist0() to add a trailing zero to a string.

This function modifies the string to add a zero after the end, and returns
the start pointer. The purpose is to use it on strings extracted by parsers
from larger strings cut with delimiters that are not important and can be
destroyed. It allows any such string to be used with regular string
functions. It's also convenient to use with printf() to show data extracted
from writable areas.