Refactor config out of two library functions

[flatcap]

The Address library is fairly self-contained; it only relies on helper functions from libmutt.

Except for two functions that rely on config variables:

• mutt_idna_local_to_intl() - $idn_encode
• mutt_idna_intl_to_local() - $idn_decode

Refactor them to pass-in a parameter so that we can remove the dependencies on libcore and libconfig.
Each function has just two callers.

mutt_idna_intl_to_local() already takes a flags parameter, that could be extended.
This flag can then be added to mutt_idna_local_to_intl().