(This path is also tracked as PR: https://github.com/WireGuard/wireguard-go/pull/56)

This change adds support for a new environment variable 'WG_KEYLOGFILE'
in resemblance to the 'SSLKEYLOGFILE' environment variable used by
curl, Chrome & Firefox to log ephemeral TLS encryption keys

When set, wireguard-go will log ephemeral keys generated during
each handshake to a file specified by the environment variable in the
WireGuard key log format.

The format used is the same as then one generated by the
extract-handshakes.sh script.

See also:
- https://git.zx2c4.com/wireguard-tools/tree/contrib/extract-handshakes
- https://wiki.wireshark.org/WireGuard#key-log-format
- https://everything.curl.dev/usingcurl/tls/sslkeylogfile

Signed-off-by: Steffen Vogel post@steffenvogel.de