BUG/MAJOR: ssl: fix a regression in ssl_sock_shutw()

BUG/MAJOR: ssl: fix a regression in ssl_sock_shutw()

Commit 405ff31 ("BUG/MINOR: ssl: assert on SSL_set_shutdown with BoringSSL")
introduced a regression causing some random crashes apparently due to
memory corruption. The issue is the use of SSL_CTX_set_quiet_shutdown()
instead of SSL_set_quiet_shutdown(), making it use a different structure
and causing the flag to be put who-knows-where.

Many thanks to Jarno Huuskonen who reported this bug early and who
bisected the issue to spot this patch. No backport is needed, this
is 1.8-specific.