merge: upstream fixes

 * Convert last rfc1524_expand_command caller to use a Buffer
 * Convert rfc1524_expand_command() implementation to use struct Buffer
 * Rename mutt_buffer_rfc1524_expand_command()
 * Fix send-mode printing when expand_filename() returns 1
 * Fix compose and edit attachment symlink failure code
 * Remove unnecessary strcmp for mutt_view_attachment()
 * Remove mutt_rfc1524_expand_filename() return value and checks
 * Make sure mailcap test %s is sanitized
 * Clean up mutt_print_attachment() cleanup
 * Add NULL checks to rfc1524_free_entry()
 * Detail the documentation of %l for index_format