CLEANUP: standard: Use len2mask4 in str2mask

The len2mask4 function was introduced in commit:
70473a5f8c56d8ec2e837b9b66443dc252b24da9
which is about six years later than the commit that introduced the
str2mask function:
2937c0dd20f2f3c0065b671bbfe3fafcd8862eaf

This is a clean up in preparation for a str2mask6 function which
will use len2mask6.

src/standard.c

@@ -1022,10 +1022,8 @@ int str2mask(const char *str, struct in_addr *mask)
 
 		if (!*str || (err && *err) || (unsigned)len > 32)
 			return 0;
-		if (len)
-			mask->s_addr = htonl(~0UL << (32 - len));
-		else
-			mask->s_addr = 0;
+
+		len2mask4(len, mask);
 	}
 	return 1;
 }