merge: upstream changes

 * Fix compilation with LibreSSL <2.7.0
 * Add attributes support on color declarations
 * Wrap ssl init calls for LibreSSL too
 * Change $pgp_use_gpg_agent to default set
 * Fix gnutls tls_socket_write() to properly retry
 * Fix tunnels to also retry and write full buffer
 * Improve attachment menu for s/mime parts
 * Simplify nested smime handling in the attachment menu
 * Show top-level decoded smime text/plain parts
 * Allow descending into maildir and mh directories in file browser
 * Add an error message for <descend-directory>
 * Updated French translation
 * Create <group-chat-reply> function
 * Updated French translation
 * Add mutt_buffer_len() helper
 * Add mutt_buffer helpers for base64 conversion
 * Change auth_gss to to use buffers and the buffer pool
 * Check the base64 decoding retval in auth_gss