split is_immutable into client_may_init and client_may_update

[rjbs]

• init is for setting things while creating
• update is for setting things via an update

I think we should bring back is_immutable, sometime, to indicate that the field is guaranteed to never change.

[wolfsage]

Looks good I think. Would be nice to have tests for:

client_may_init => 1,
client_may_update => 0,

and

client_may_init => 0,
client_may_update => 1,

[rjbs]

This has been applied.