Rework Active Directory code to make use of kerberos keytabs

- Renamed ad_adminname and ad_adminpw to more generic ad_bindname and ad_bindpw
- Created necessary migration to do the rename and preserve the contents
  of the old fields
- Modified all code that references those names accordingly
- Added ad_keytab field and ad_use_keytab to specify using a keytab
- Modified ix-kinit to use kerberos keytab if present
- ad_bindname and ad_bindpw are overloaded now, if not using a keytab, they
  work like ad_adminname and ad_adminpw did. Administrator credentials
  can be used. If using a keytab, a less privileged user account should
  be used that has sufficient privilege to perform LDAP queries. The
  keytab used should have privileges to grant tickets for cifs and ldap
  (host/*) should work. A keytab can be created on windows like so:

  ktpass.exe -out hostname.keytab -princ host/hostname@DOMAINNAME -ptype
      KRB5_NT_PRINCIPAL -mapuser DOMAIN\username -pass userpass

  setspn -A host/hostname@DOMAINNAME DOMAIN\username

  If done correctly, once the keytab is generated, it can be uploaded to
  FreeNAS through the GUI and then click on the option to use a keytab
  and Active Directory joins will be passwordless ;-)

Ticket: #1403