scan config file for unsupported tags

[lightning-auriga]

as of !35, unsupported tags in --config/-c are silently ignored. this isn't necessarily a bad thing, but leads to confusing behavior if the config was created based on an old version of the interface specification. possible solutions:

• scan config in c++ and error on unrecognized tags (probably not desirable)
• scan config in c++ and warn on unrecognized tags (decent behavior but cumbersome coding)
• provide json schema specifying config format and suggest user run python3 validate (certainly more convenient for me but requires user effort)

[lightning-auriga]

so I think I'm going to compromise and invoke validate via an exec call after minimal command line parsing.

[lightning-auriga]

this should be fully patched in as of 92c2003 and will hopefully prevent much confusion in the future.