Cleanup and improve `tinc fsck`.

Cleanup and improve `tinc fsck`.

- implement TODOs
- fix an invalid warning:
	WARNING: public and private RSA keys do not match
- use the same configuration reading & parsing logic as in tincd
- read keys from all supported variables
- auto fix a few more broken key configurations
- fix a couple of rare memory leaks
- add warnings for host variables in server config and vice versa
- check duplicates for all configuration variables (not the first 50)
- check_conffile had a stack-buffer-underflow with going before the start of the line