src: don't crash on incomplete user-edited config file

If a user created a custom config file and we added new fields in a
newer version, `eg25-manager` will crash by assuming all config files
are complete and up-to-date.

Moreover, creating a custom config, even to change only one option,
required copying a complete default config file and then editing the
relevant field(s). This could lead to issues when upgrading, as default
values of fields unrelated to the user change could be modified and not
applied to the user config.

This patch addresses both these issues by:
* making sure at least one config file exists
* requiring only the "default" config file (the one under `/usr/share`)
  to include all the required fields
* trying to use user config for each field, falling back to the default
  config file if the field isn't present in the user config
* exiting with a meaningful error message in case the default config
  file is missing a required field or section

That way, it will be possible to have a minimal user config file
containing only the field(s) needing a different value than the default
one, falling back to the values in the default config file.

Fixes #23

src/suspend.h

@@ -8,7 +8,7 @@
 
 #include "manager.h"
 
-void suspend_init (struct EG25Manager *data, toml_table_t *config);
+void suspend_init (struct EG25Manager *data, toml_table_t *config[]);
 void suspend_destroy (struct EG25Manager *data);
 
 void suspend_inhibit (struct EG25Manager *data, gboolean inhibit, gboolean block);