[Info-vax] Backup API mailbox oddity

[osuvman50 at gmail.com]

On Thursday, January 11, 2018 at 3:55:22 PM UTC-5, I wrote:
> Building the control structure with large list of input files isn't really 
> that difficult in C, but it would be easier if the API had a no-op flag
> structure to use as padding or a chain-to address option.

I tried a version that replaced the mailbox with the file list directly in the
control block as BCK_OPT_K_INPUT structures.  The initialization portion of
backup$start() exhibits O(n^3) scaling as it rebuilds the file list internally.
The internal list is a chain of context blocks (for RMS $SEARCH processing)
without a tail pointer so it has to walk the chain to the end for every block
added.  Not a bad choice for a handful of file specifications on a command line, but bad news for a 10,000 file list.