config: add --validate option

running `lb config --validate` causes the script to stop after running
the validation check on the config compiled at that point, prior to
writing the config to disk.

this gives users the ability to check the validity of a config without
modifying or rewriting the saved config.

note that if users provide new config options alongside --validate, these
are taken into account in the check performed.

the 'check complete' message will not be seen if an error is reported by
the check function, while it will be seen if only warnings are given, but
it would require a redesign of the validation check function to make any
improvement in that area, and it's perhaps not worth it.

Gbp-Dch: Short

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