Honour the source-port when retrying in dns_dispatch

When retrying in the DNS dispatch, the local port would be forgotten on ISC_R_ADDRINUSE, keep the configured source-port even when retrying.

Additionally, treat ISC_R_NOPERM same as ISC_R_ADDRINUSE.

Closes: #3986 (closed)

Edited by Ondřej Surý

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