Replace input values with input names when logging to syslog

This substitution works very well for bash, but it might look odd in the output of a Ruby or PowerShell script.

When we filter credential values in the core, we use this substitution:
    app/biz/audit_entry_mixin.rb:    MASKED_CRED_TEXT       = '<hidden credential>'

Maybe we should use a similar substitution in this case:
    "<hidden input #{k}>"

This will result in a consistent appearance for all filtered things -- it's important, since the RightLink filtering will now happen before the server-side filtering and therefore it will prevent server-side filtering from doing anything; woo hoo!