https://fedorahosted.org/sssd/ticket/1808

- Use a double-linked list for domains and subdomains.
- Never remove a subdomain, simply mark it as disabled if it becomes
  unused.
- Rework the way subdomains are refreshed.
  Now sysdb_update_subdomains() actually updates the current subdomains
  and marks as disabled the ones not found in the sysdb or add new ones
  found. It never removes them.
  Removal of missing domains from sysdb is deferred to the providers,
  which will perform it at refresh time, for the ipa provider that is
  done by ipa_subdomains_write_mappings() now.
  sysdb_update_subdomains() is then used to update the memory hierarchy
  of the subdomains.

- Removes sysdb_get_subdomains()
- Removes copy_subdomain()
- Add sysdb_subdomain_delete()

Use this function instead of explicitly calling domain->next

This function allows to get the next primary domain or to descend into the
subdomains and replaces also get_next_dom_or_subdom()

When read from the domain section, the pwd_expiration_warning was
properly converted to seconds from days, but not the
pam_pwd_expiration_warning set in the [pam] section.

https://fedorahosted.org/sssd/ticket/1773

A sysdb contains now multiple domains, but the mpg property is a
property of a specific domain not of the underlying database.

Added new parameter to split_on_separator that allows to skip
empty values.

The whole function was rewritten. Unit test case was added to
check the new implementation.

https://fedorahosted.org/sssd/ticket/1484