QUIC: split keys availability checks to read and write sides. Keys may be released by TLS stack in different times, so it makes sense to check this independently as well. This allows to fine-tune what key direction is used when checking keys availability. When discarding, server keys are now marked in addition to client keys.

# Copyright (C) Igor Sysoev
# Copyright (C) Nginx, Inc.


cat << END >> $NGX_AUTO_HEADERS_H

#ifndef $have
#define $have  1
#endif

END