net/dns: fix openresolv DNS config when no other snippets exist (#20996)

openresolv exits 2 when no config snippets are registered; treating that
as a failure aborted the entire DNS reconfiguration. Filtering out our own
snippet could also leave "resolvconf -l" with no arguments, which lists
every snippet including ours and would make quad-100 its own upstream.

Fixes #20825

Signed-off-by: Brendan Creane <bcreane@gmail.com>