* Introduce new system of configuration checks:

* Introduce new system of configuration checks:
 - now symbols inside metrics definition must be inside rules as well
 - symbols may be virtual (e.g. when module can insert several symbols inside callback)
 - symbols may be pure callbacks (when symbol's name is unknown and depends on conditions)
* Module 'emails' is removed as it is not used in the current rspamd
MANY fixes to sample config files