Add "cluster introducer" functionality to nodes (ref #120)

protocol/PROTOCOL.md

@@ -249,7 +249,7 @@ The Node Flags field contains the following single bit flags:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-    |          Reserved         |Pri|          Reserved         |R|T|
+    |          Reserved         |Pri|          Reserved       |I|R|T|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
  - Bit 31 ("T", Trusted) is set for nodes that participate in trusted
@@ -258,6 +258,9 @@ The Node Flags field contains the following single bit flags:
  - Bit 30 ("R", Read Only) is set for nodes that participate in read
    only mode.
 
+ - Bit 29 ("I", Introducer) is set for nodes that are trusted as cluster
+   introducers.
+
  - Bits 16 through 28 are reserved and MUST be set to zero.
 
  - Bits 14-15 ("Pri) indicate the node's upload priority for this
@@ -276,7 +279,7 @@ The Node Flags field contains the following single bit flags:
 
  - Bits 0 through 14 are reserved and MUST be set to zero.
 
-Exactly one of the T, R or S bits MUST be set.
+Exactly one of the T and R bits MUST be set.
 
 The per node Max Local Version field contains the highest local file
 version number of the files already known to be in the index sent by

protocol/protocol.go

@@ -47,6 +47,7 @@ const (
 const (
 	FlagShareTrusted  uint32 = 1 << 0
 	FlagShareReadOnly        = 1 << 1
+	FlagIntroducer           = 1 << 2
 	FlagShareBits            = 0x000000ff
 )