Tracking entry for a VMS specific issue in util_str.c/string_sub()

The $ character has a different meaning in OpenVMS DCL shell commands than with
UNIX, and substituting an underscore for it can cause failures.

In OpenVMS, the presence of a $ in a filename, environment symbol, or other
system resource name is used to indicate that the use of that name is reserved
to OpenVMS designated uses, and it prevents name space collisions.

Of course because the bash shell is optionally available on OpenVMS,
consideration may be needed to prevent exploits, if this is what the string
substitution is suppose to prevent.