tls: write SSLKEYLOGFILE line in one syscall
tls: write SSLKEYLOGFILE line in one syscall

 * Write the key log line and LF in one go to avoid problems with
   multiple interleaving applications.
 * Drop the "QUIC_" prefix, since draft-ietf-quic-tls-17 the same HKDF
   label from TLS 1.3 is in use. Wireshark v3.1.0rc0-836-gcc50ec3634
   supports both, but the "QUIC_" variant is deprecated.

cloudflare/quiche,cloudflare/quiche,cloudflare/quiche