net: use a secure cipher list by default

Previously, the `net` module use the blanket "ALL" as the default cipher
list. This list may contain security ciphers that are weak and/or outdated
according to the current standard.

This commit introduces a new module `ssl_config` that contains the
latest OpenSSL configurations as recommended by Mozilla OpSec, and
make the `net` module use the cipher list targeting `intermediate`
compatibility level as the default.

lib/pure/net.nim

@@ -68,6 +68,7 @@ import std/private/since
 
 import nativesockets, os, strutils, times, sets, options, std/monotimes
 from ssl_certs import scanSSLCertificates
+import ssl_config
 export nativesockets.Port, nativesockets.`$`, nativesockets.`==`
 export Domain, SockType, Protocol
 
@@ -533,7 +534,7 @@ when defineSsl:
         raiseSSLError("Verification of private key file failed.")
 
   proc newContext*(protVersion = protSSLv23, verifyMode = CVerifyPeer,
-                   certFile = "", keyFile = "", cipherList = "ALL",
+                   certFile = "", keyFile = "", cipherList = CiphersIntermediate,
                    caDir = "", caFile = ""): SSLContext =
     ## Creates an SSL context.
     ##