Improve get_roster command result: show groups as a list

[badlop]

This PR improves the get_roster command, and fixes ejabberdctl to fully support that command and other ones like that.

This changes the API, so the release notes should describe it. For example:

---

Changes in get_roster command

There are some changes in the result output of the get_roster command defined in mod_admin_extra:

• ask is renamed to pending
• group is renamed to groups
• the new groups is a list with all the group names
• a contact that is in several groups is now listed only once, and the groups are properly listed.

For example, let's say that admin@localhost has two contacts: a contact is present in two groups (group1 and group2), the other contact is only present in a group (group3).

The old get_roster command in ejabberd 23.04 and previous versions was like:

$ ejabberdctl get_roster admin localhost
jan@localhost jan   none    subscribe       group1
jan@localhost jan   none    subscribe       group2
tom@localhost tom   none    subscribe       group3

The new get_roster command in ejabberd 23.XX and newer versions returns as result:

$ ejabberdctl get_roster admin localhost
jan@localhost jan   none    subscribe       group1;group2
tom@localhost tom   none    subscribe       group3

Notice that the ejabberdctl command-line tool since now will represent list elements in results separated with ;

[coveralls]

coverage: 32.774% (-0.005%) from 32.779% when pulling 245c9ae on badlop:get-roster-command into 426fd14 on processone:master.

[Neustradamus]

Excellent!