Add support for multiple types.db files

__Proposal:__
Support for multiple types.db files in the "typesdb" = specification

__Current behavior:__

A single value is supported, which can be either a file or a directory (in which case the contents of the directory are read in as .db files

__Desired behavior:__

Multiple values supported.

__Use case:__

This is useful when packaging -- for instance even when packaging collectd along is common to allow for a global package controlled types.db file as well as a user override in a different location (outside the scope of directories controlled by the packaging system). This is multiplied as soon as there are multiple packages that provide valid types.db files.

Start from the `typesdb` specification and trace how its current single file-or-directory value is interpreted. Review the collectd packaging use case described in the issue, and consider how multiple values should behave across global and user-provided files. Done means multiple `types.db` paths are accepted while preserving existing directory behavior.

Written by the indexing model from the issue text.