schannel: support CALG_ECDH_EPHEM algorithm

Add support for Ephemeral elliptic curve Diffie-Hellman key exchange
algorithm option when selecting ciphers. This became available on the
Win10 SDK.

Closes https://github.com/curl/curl/pull/3608

docs/CIPHERS.md

@@ -496,3 +496,4 @@ WinSSL allows the enabling and disabling of encryption algorithms, but not speci
 `CALG_ECDH`,
 `CALG_ECMQV`,
 `CALG_ECDSA`,
+`CALG_ECDH_EPHEM`,

lib/vtls/schannel.c

@@ -323,6 +323,9 @@ get_alg_id_by_name(char *name)
 #endif
 #ifdef CALG_ECDSA
   CIPHEROPTION(CALG_ECDSA);
+#endif
+#ifdef CALG_ECDH_EPHEM
+  CIPHEROPTION(CALG_ECDH_EPHEM);
 #endif
   return 0;
 }