Backport some IO enums from newio branch

src/core/IO.pm

@@ -6,6 +6,32 @@ my role IO {
     method umask { state $ = :8( qx/umask/.chomp ) }
 }
 
+enum SeekType (
+  :SeekFromBeginning(0),
+  :SeekFromCurrent(1),
+  :SeekFromEnd(2),
+);
+enum ProtocolFamily (
+  :PF_LOCAL(0),
+  :PF_UNIX(1),
+  :PF_INET(2),
+  :PF_INET6(3),
+  :PF_MAX(4),
+);
+enum SocketType (
+  :SOCK_PACKET(0),
+  :SOCK_STREAM(1),
+  :SOCK_DGRAM(2),
+  :SOCK_RAW(3),
+  :SOCK_RDM(4),
+  :SOCK_SEQPACKET(5),
+  :SOCK_MAX(6),
+);
+enum ProtocolType (
+  :PROTO_TCP(6),
+  :PROTO_UDP(17),
+);
+
 #===============================================================================
 # Primitives that may live in a role someday, if we figure out how to do that
 # in a sensible and performant way.  Until then, these global subs with slightly