Fix duplicate local printers in DNS-SD discovery (Issue #1531)

[abubakarsabir924-cell]

Fixes #1531

Root cause: In cups_enum_dests(), permanent cupsd queues were
never stored in _cups_dnssd_data_t. So when cups_dest_query_cb()
fired, there was no local queue data to compare against DNS-SD
discovered printers.

Fix:

• Added num_local and local_dests fields to _cups_dnssd_data_t
• Store permanent cupsd queues before DNS-SD browse starts
• In cups_dest_query_cb(), compare rp field from DNS-SD TXT
  record against local queue names
• If matched, mark device as INCOMPATIBLE to suppress duplicate

Note: Tested build successfully. Would appreciate testing from
someone who can reproduce the issue with a shared printer setup.

[abubakarsabir924-cell]

@tillkamppeter Sir,
I found the exact root cause: permanent cupsd queues were never stored in _cups_dnssd_data_t, so cups_dest_query_cb() had no local data to compare against DNS-SD discovered printers.

The fix compares the rp field from DNS-SD TXT records against local queue names — if matched, the device is marked INCOMPATIBLE to suppress the duplicate.
I chose rp over UUID because rp is available directly at TXT parse time, avoiding async timing issues.

I tested the build successfully and ran lpstat -e with a local test printer — no duplicates appeared. However, I don't have a shared printer setup to fully reproduce the exact scenario from the issue, so I'd appreciate if you could test on your system.

I don't have a shared printer setup to reproduce the exact scenario locally. Since you mentioned you can reproduce this, could you please test this fix on your system?

Happy to update based on your feedback!

[abubakarsabir924-cell]

Hi @tillkamppeter Sir, I hope you're doing well.

I'm just following up on this PR when you have a moment to review, thank you

[michaelrsweet]

LGTM

[abubakarsabir924-cell]

@michaelrsweet Sir,

Thank you so much for all your guidance for all issues.