This code contained a bug where any argument to /HISTORY that _didn't_ start with "-CL" would clear history,
so no number or pattern argument could successfully be passed.
With this fix only an argument that case-insensitively matches "-clear" will clear history, so a pattern
argument will also work correctly.
sindex(), where neither argument is NULL and the accept string does not start with ^, is exactly equivalent
to the standard function strpbrk().
Further, strpbrk() where the accept string is only one character long, is exactly equivalent to strchr().
