merge: upstream fixes (mutt/default)

 * Add SNI support for OpenSSL. (see #3923)
 * Add SNI support for GnuTLS. (closes #3923)
 * Add shortcuts for IMAP and POP mailboxes in the file browser
 * Change OpenSSL to use SHA-256 for cert comparison. (closes #3924)
 * Fix conststrings type mismatches. (closes #3926)
 * Pass envlist to filter children too.  (closes #3922)
 * Fix mutt_envlist_set() for the case that envlist is null. (see #3922)
 * Fix setenv overwriting to not truncate the envlist. (see #3922)