[#328] Introduce subcommand config-get.

[fluca1978]

This commit introduces the pgagroal-cli subcommand config-get that allows the user to query the running pgagroal instance and obtain the value of a configuration parameter.

A new management action, named ACTION_CONFIG_GET, is added to manage the communication over the socket between pgagroal-cli and the main pgagroal process.
A few utility functions to handle the communication have been introduced to send data over the socket and receive it on the socket.

Moreover, different utility functions to print out the value of a configuration setting have been introduced, so that for instance all the boolean values are always mapped to "on" and "off".

The pgagroal-cli config-get command accepts a key to search. Usually the key is searched into the global configuration namespace, that is the section [pgagroal]. However, it is possible to define other keys that can be handled:

• server.<server-name>.<what> will search 'what' within the server section named [server-name], as an example server.venkman.host;
• limit.<database>.<what> will search 'what' within the pgagroal_databases.conf file entries (runtime) for the specified database name;
• hba.<user>.<what> will search 'what' among the runtime HBA configuration for the user 'user'.

Note that in the case of 'hba.' and 'limit.', since the username or database name can appear multiple times, only the first occurency is matched. The match happens in a simple way, so for instance the database name 'foo' does not match an entry with 'all'.

It is also possible to specify a key starting with pgagroal., that will instrument the search within the global namespace. Therefore max_entries and pgagroal.max_entries are the same search terms.

If a key is not found, the main pgagroal process issue a warning in the logs, while the application pgagroal-cli does receive an empty answer and therefore does nothing.
Even in the case of a non-existing key, pgagroal-cli exists normally, to indicate that the query has been performed well, simply with no meaningful answer.

If pgagroal-cli is running with --verbose flag, the application prints the configuration entry with the key and the value, like in

pgagroal-cli get-config max_entries
max_entries = 300

this is useful for humans, but not for automated systems, so the default way of answering a query is by printing only the value

pgagroal-cli get-config max_entries
300

Documentation updated.
Shell completions updated.

Close #328

[jesperpedersen]

Rebased, and merged.

Thanks for your contribution !