MINOR: ssl/log: add keylog format variables and env vars

MINOR: ssl/log: add keylog format variables and env vars

Add keylog_format_fc and keylog_format_bc global variables containing
the SSLKEYLOGFILE log-format strings for the frontend (client-facing)
and backend (server-facing) TLS connections respectively. These produce
output compatible with the SSLKEYLOGFILE format described at:
https://tlswg.org/sslkeylogfile/draft-ietf-tls-keylogfile.html

Both formats are also exported as environment variables at startup:
  HAPROXY_KEYLOG_FC_LOG_FMT
  HAPROXY_KEYLOG_BC_LOG_FMT

These variables contains \n so they might not be compatible with syslog
servers, using them with stderr or a sink might be required.

These can be referenced directly in "log-format" directives to produce
SSLKEYLOGFILE-compatible output, usable by network analyzers such as
Wireshark to decrypt captured TLS traffic.