The major goal is to simplify certificate management in ejabberd.
Currently it requires some effort from a user to configure certficates,
especially in the situation where a lot of virtual domains are hosted.
The task is splitted in several sub-tasks:
* Implement basic certificate validator. The validator should check all
configured certificates for existence, validity, duration and so on. The
validator should not perform any actions in the case of errors except
logging an error message. This is actually implemented by this commit.
* All certificates should be configured inside a single section (something
like 'certfiles') where ejabberd should parse them, check the full-chain,
find the corresponding private keys and, if needed, resort chains and
split the certficates into separate files for easy to use by fast_tls.
* Options like 'domain_certfile', 'c2s_certfile' or 's2s_certfile' should
probably be deprecated, since the process of matching certificates with the
corresponding virtual hosts should be done automatically and these options
only introduce configuration errors without any meaningful purpose.
The changes are very similar to those from previous commit:
* Now there is no need to pass validating function in
gen_mod:get_opt() and gen_mod:get_module_opt() functions,
because the modules' configuration keeps already validated values.
* New functions gen_mod:get_opt/2 and gen_mod:get_module_opt/3 are
introduced.
* Functions gen_mod:get_opt/4 and get_module_opt/5 are deprecated.
If the functions are still called, the "function" argument is
simply ignored.
* Validating callback Mod:listen_opt_type/1 is introduced to validate
listening options at startup.
