add security level bindings

[jmayclin]

This PR adds bindings for a limited set of the set_security_level methods

void SSL_CTX_set_security_level(SSL_CTX *ctx, int level);
void SSL_set_security_level(SSL *s, int level);

int SSL_CTX_get_security_level(const SSL_CTX *ctx);
int SSL_get_security_level(const SSL *s);

https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_security_level.html

These functions were added in OpenSSL 1.1.0.

SSL_CTX_set_security_level() and SSL_set_security_level() do not return values.
SSL_CTX_get_security_level() and SSL_get_security_level() return a integer that represents the security level with SSL_CTX or SSL, respectively.

I chose to expose the security level as an unsigned integer, even through the C api uses a signed integer because the only described security levels are 0 - 5 inclusive. This follows the precedent for the set_verify_depth bindings which similarly uses a u32 in the rust bindings even through the C api uses a signed integer. Let me know if you'd prefer the security level to be represented as an i32 instead.

This PR addresses #1380

[botovq]

Setting the security level is already possible via set_cipher_list and set_ciphersuites using the @SECLEVEL=n cipher string.