Remove buggy usage of M_ICONV_HOOK_TO flag in mutt_idna.c:mutt_idna_to_local().

Remove buggy usage of M_ICONV_HOOK_TO flag in mutt_idna.c:mutt_idna_to_local().
Wipe unwanted code depending on M_ICONV_HOOK_TO in charset.c:mutt_iconv_open().
Totally wipe M_ICONV_HOOK_TO symbol.
Remove misusages of M_ICONV_HOOK_FROM flag in:
 - crypt-gpgme.c:print_utf8().
 - mutt_idna.c:mutt_idna_to_local() and mutt_local_to_idna().
 - pgp.c:pgp_traditional_encryptsign().
Document usage policy of M_ICONV_HOOK_FROM flag.
Cosmetic downcasing of some constant charset names (utf-8, euc-jp) for consistency.
Correction of a typo in the "iso8859-5" charset name.