Server Entry Point: Does Character by name X exist

[brightrim]

It would be very useful to have a server entry point that can check whether an entered string of text actually matches a character name in the database, returning a boolean value.

Examples of where this would be used:
Housing -> When setting the tenant/guest/builder-permission names (Functions already exists on housing test server, but allows typing names of non-existent characters currently)
GM tools -> When scheduling changes to quest statuses/other things for an offline players next login (Function already exists on live but allows typing names of non-existent characters currently)
In game messengers -> When typing in the recipient for your message (Not yet implemented, ideally awaiting this server entry point before implementation)

[vilarion]

Implemented the following in 669a8ec:
exists, id = world:getPlayerIdByName(name)
exists: true iff player with name exists in the database
id: id of the player with name, if that player exists

Still needs to be documented: Illarion-eV/Illarion-Lua#14