merge: upstream refactoring

 * Properly revert on an autocrypt postpone failure
 * Fix memory leak when attaching messages
 * Convert bcache to use buffer pools
 * Convert mutt_complete() to use the buffer pool
 * Convert compose check_attachments() to use buffer pool
 * Fix mutt_save_message() "enter_fname()" to use buffer version
 * Create mutt_buffer_get_field()
 * Convert buffer callers of mutt_get_field() to use buffer version
 * Convert compose menu fname variable to buffer
 * Convert compress setup_paths() to use buffer
 * Fix memory leak in compose write-fcc function
 * buffer: tidy checks for empty Buffers