In order to use passphrase-protected keys.
This patch does not change the default behavior:

    server.useSSL("./cert/server.crt", "./cert/server.key");
    $> ./a.out
    Enter PEM pass phrase:
    [...]

    static int password_callback(char *buf, int size, int rwflag, void *u)
    {
        static const char *password = "foobar"; // _Please_ don't do that, that's an example
        strncpy(buf, password, size);
        return strlen(password);
    }

    server.useSSL("./cert/server.crt", "./cert/server.key", false, &password_callback);
    $> ./a.out
    Listening on 0.0.0.0:9080

Key has been generated with:

    $> openssl genrsa -aes128 -passout pass:foobar -out server.key 3072

If you intend to use this feature, please look at the recent
developments on memfd_secret[1] in order to actually store a passphrase
with relative security in memory.

[1] https://lwn.net/Articles/865256/Signed-off-by: Louis Solofrizzo <lsolofrizzo@scaleway.com>