Require the master key when performing certain operations (RIPD-666):
* When disabling the use of the master key; or
* When enabling 'no freeze'.

Additionally, a small fix in the handling of 'sfMessageKey' which
would prevent the field from being unset, and some small cleanups.
[FOLD] Require master key to set NoFreeze
[FOLD] Fix missing 'else' when handling sfMessageKey
[FOLD] Cleanup handling a little bit