merge: refactor to reduce variable scope

These changes should not affect the behaviour of the code.

 * init.c
 * main.c
 * mbox.c
 * menu.c
 * mh.c
 * mutt/base64.c
 * mutt/charset.c
 * mutt/file.c
 * mutt/hash.c
 * mutt/rfc2047.c
 * mutt_notmuch.c
 * muttlib.c
 * mx.c
 * ncrypt/crypt.c
 * ncrypt/crypt_gpgme.c
 * ncrypt/pgp.c
 * ncrypt/smime.c