lib-smtp: smtp-address - Implement workarounds for parsing addresses with a bad local...

lib-smtp: smtp-address - Implement workarounds for parsing addresses with a bad localpart.

Always parse it as normal when it starts with `<' or `"'. The workaround
consists of parsing the address by splitting it on the last `@'. The address is
accepted when the resulting localpart and domain can be used to compose a valid
address. This workaround is now shared with username-to-address parsing, which
existed before.