[#26] Use SSL_CTX_use_certificate_chain_file

(reference #26)

Via @mannol

```
Currently, libevhtp is using SSL_CTX_use_certificate_file to load a
certificate file. That function lacks the ability to load the pinned
certificate chain (if any) which has a consequence of connecting clients
not trusting the received certificate. By using
SSL_CTX_use_certificate_chain_file we give the libssl the ability to
read and send the entire certificate chain (if any), which clients can
check against.
```

evhtp.c

@@ -4878,7 +4878,7 @@ evhtp_ssl_init(evhtp_t * htp, evhtp_ssl_cfg_t * cfg)
             break;
     }     /* switch */
 
-    SSL_CTX_use_certificate_file(htp->ssl_ctx, cfg->pemfile, SSL_FILETYPE_PEM);
+    SSL_CTX_use_certificate_chain_file(htp->ssl_ctx, cfg->pemfile);
 
     char * const key = cfg->privfile ?  cfg->privfile : cfg->pemfile;