proposal: Handle skipping DH groups directly in select() and matches()

Also renames the flag.

src/libstrongswan/crypto/proposal/proposal.c

@@ -434,6 +434,10 @@ static bool select_algos(private_proposal_t *this, proposal_t *other,
 		{
 			continue;
 		}
+		if (type == DIFFIE_HELLMAN_GROUP && (flags & PROPOSAL_SKIP_DH))
+		{
+			continue;
+		}
 		if (select_algo(this, other, type, flags, selected != NULL, &alg, &ks))
 		{
 			if (alg == 0 && type != EXTENDED_SEQUENCE_NUMBERS)

src/libstrongswan/crypto/proposal/proposal.h

@@ -60,8 +60,8 @@ enum proposal_selection_flag_t {
 	PROPOSAL_ALLOW_PRIVATE = (1<<0),
 	/** Whether to prefer configured or supplied proposals. */
 	PROPOSAL_PREFER_CONFIGURED = (1<<1),
-	/** Whether to strip out diffie hellman groups */
-	PROPOSAL_STRIP_DH = (1<<2),
+	/** Whether to skip and ignore diffie hellman groups. */
+	PROPOSAL_SKIP_DH = (1<<2),
 };
 
 /**