CLEANUP: Use the ist() macro whenever possible

CLEANUP: Use the ist() macro whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    char *s;
    @@

    (
    - ist2(s, strlen(s))
    + ist(s)
    |
    - ist2(strdup(s), strlen(s))
    + ist(strdup(s))
    )

Note that this replacement is safe even in the strdup() case, because `ist()`
will not call `strlen()` on a `NULL` pointer. Instead is inserts a length of
`0`, effectively resulting in `IST_NULL`.