Make the 'debug' configuration option more fine-grained.

Some things, like logging of SAML messages or backtraces, are controlled with the 'debug' configuration option. However, it might be possible that we don't want one while we want the other, but that's impossible with just one option.

This commit allows us to configure debugging options independently, but groupping all of them together. This is particularly useful if we want to log backtraces to debug errors, for example, but we don't want to log SAML messages to keep the privacy of the users. This also allows us to get rid of the 'debug.validatexml' configuration option, and group it with other debug options.

This changes are backwards-compatible. Old and new configurations will work at the same time.