cnid_dbd: Set explicit max length of db_params to prevent buffer overflow

[rdmark]

Security vulnerability flagged by Sonarcloud and picked up by GitHub.

https://github.com/Netatalk/netatalk/security/code-scanning/2

[rdmark]

Tested that each type of db_param can be parsed properly:

Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: read key:logfile_autoremove val:0
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: setting logfile_autoremove to 0
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: read key:cachesize val:10000
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: setting cachesize to 10000
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: read key:flush_frequency val:600
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: setting flush_frequency to 600
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: read key:flush_interval val:800
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: setting flush_interval to 800
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: read key:fd_table_size val:1024
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: setting max number of concurrent afpd connections per volume (fd_table_size) to 1024
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: read key:idle_timeout val:200
Sep 22 17:11:14 alien cnid_dbd[1378150]: db_param: setting idle timeout to 200

[sonarcloud]

Quality Gate failed

Failed conditions
E Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

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[rdmark]

The Sonar detected bug is not new, but rather a long-standing one that was unearthed here. Will address in the future.