Description of problem:
Since RHEL-7.3, dovecot-2.2.10-7 changed default value first_valid_uid
from 500 to 1000, because of regular users on rhel-7 start at UID 1000.
See bug #1280433

We currently inform users about different ranges for system,reserved and reg.
users on rhel-7 according to ranges on rhel-6. However, this is another change
and as I remember, we don't move users to different UIDs - as this is
completely in hands of admins now.

So after upgrade to rhel-7, dovecot will not be usable for all users with lower UID then used first_valid_uid.

Known issue:
For bug #1280433 will be created known issue, which will include upgrades from rhel-6 too.


Expected solution:
* in case the first_valid_uid is set, we don't have to do anything, but we could print log_info probably that the default value is changed - just for theirs info

* in case the first_valid_uid is not set:
  - should print slight risk or info,
  - exit status will be fixed or failed (it depends on another already created checks of the module, important part is, that exit will not be passed),
  - the postupgrade script will set "first_valid_uid=500" inside
    /etc/dovecot/conf.d/10-mail.conf