Change nsec3param salt config to saltlen

Change nsec3param salt config to saltlen

Upon request from Mark, change the configuration of salt to salt
length.

Introduce a new function 'dns_zone_checknsec3aram' that can be used
upon reconfiguration to check if the existing NSEC3 parameters are
in sync with the configuration. If a salt is used that matches the
configured salt length, don't change the NSEC3 parameters.

(cherry picked from commit 6f97bb6b1fae8031d9c6a1eb699d463a7369e672)