Make spec docs reference RFC2119

Now people will have no excuse for not knowing what "MUST" and "MAY"
mean.  Fixes bug 1310.

doc/spec/control-spec.txt

@@ -15,6 +15,11 @@
   versions 0.1.0.x; the protocol in this document only works with Tor
   versions in the 0.1.1.x series and later.)
 
+      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+      "OPTIONAL" in this document are to be interpreted as described in
+      RFC 2119.
+
 1. Protocol outline
 
   TC is a bidirectional message-based protocol.  It assumes an underlying

doc/spec/dir-spec.txt

@@ -18,13 +18,15 @@
        103  Splitting identity key from regularly used signing key
        104  Long and Short Router Descriptors
 
-   AS OF 14 JUNE 2007, THIS SPECIFICATION HAS NOT YET BEEN COMPLETELY
-   IMPLEMENTED, OR COMPLETELY COMPLETED.
-
    XXX when to download certificates.
    XXX timeline
    XXX fill in XXXXs
 
+      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+      "OPTIONAL" in this document are to be interpreted as described in
+      RFC 2119.
+
 0.1. History
 
    The earliest versions of Onion Routing shipped with a list of known

doc/spec/path-spec.txt

@@ -14,6 +14,11 @@ of their choices.
 
                     THIS SPEC ISN'T DONE YET.
 
+      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+      "OPTIONAL" in this document are to be interpreted as described in
+      RFC 2119.
+
 1. General operation
 
    Tor begins building circuits as soon as it has enough directory

doc/spec/rend-spec.txt

@@ -3,6 +3,11 @@
 
 0. Overview and preliminaries
 
+      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+      "OPTIONAL" in this document are to be interpreted as described in
+      RFC 2119.
+
    Read
    https://www.torproject.org/doc/design-paper/tor-design.html#sec:rendezvous
    before you read this specification. It will make more sense.

doc/spec/tor-spec.txt

@@ -16,6 +16,11 @@ see tor-design.pdf.
 
 0. Preliminaries
 
+      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+      "OPTIONAL" in this document are to be interpreted as described in
+      RFC 2119.
+
 0.1.  Notation and encoding
 
    PK -- a public key.