Overhaul of "doc" build target for generating policy script documentation.

It's now all implemented in CMake scripting.

The generation of reST docs is now a distinct target, "restdoc", while
the target to generate HTML docs, "doc", depends on "restdoc".  reST doc
generation supports incremental builds (documentation for a given policy
script is only regenerated when it is out of date), but HTML doc generation
via ``make doc`` is not incremental (Sphinx always starts with fresh input).

Building the "restdoc" target is now covered by a btest to ensure all
policy scripts are parse-able when Bro is in "doc mode".

Generated reST docs should now support "@load"ing from subdirectories.  e.g.
"@load foo/baz" and "@load bar/baz" will now generate the right xref links.

testing/btest/Baseline/doc.autogen-reST-example/example.rst

@@ -17,7 +17,7 @@ these comments are transferred directly into the auto-generated
 
 :Author: Jon Siwek <jsiwek@ncsa.illinois.edu>
 
-:Imports: :doc:`notice`
+:Imports: :doc:`notice </policy/notice>`
 
 Summary
 ~~~~~~~
@@ -234,8 +234,8 @@ Port Analysis
 SSL::
 
     [ports={
-        563/tcp,
-        443/tcp
+        443/tcp,
+        562/tcp
     }]
 
 Packet Filter
@@ -244,8 +244,8 @@ Packet Filter
 
 Filters added::
 
-    [nntps] = tcp port 563,
-    [ssl] = tcp port 443
+    [ssl] = tcp port 443,
+    [nntps] = tcp port 562
 
 Private Interface
 -----------------
@@ -260,8 +260,8 @@ State Variables
    ::
 
       {
-         563/tcp,
-         443/tcp
+         443/tcp,
+         562/tcp
       }
 
 Types

testing/btest/doc/autogen-reST-all

@@ -0,0 +1 @@
+@TEST-EXEC: cd $BUILD && make restdoc

testing/btest/doc/autogen-reST-example

@@ -1,2 +1,2 @@
 @TEST-EXEC: bro --doc-scripts $DIST/doc/scripts/example.bro
-@TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/doc/example-diff-canonifier.py btest-diff example.rst
+@TEST-EXEC: btest-diff example.rst