VMM: VirtualMailManager.__pwhash() added support for 'doveadm pw'.
dovecotpw was replaced by `doveadm pw` in Dovecot v2.0. To use doveadm
instead of dovecotpw use the following settings in vmm.cfg:
    [bin]
    dovecotpw = /path/to/doveadm
    [misc]
    dovecotvers = 20
- Aliases
    - avoid looping aliases
- Domain
    - optional limits for number of:
        + accounts
        + aliases
        + destinations/alias
        + alias domains