Install zkg as part of the Zeek distribution.

- Add auxil/package-manager submodule as an optional build
  source. When the submodule is present, zkg gets installed into the
  Zeek installation's bin directory, its config file into etc/zkg, and
  its state into var/lib/zkg. Like zeekctl, zkg finds its own module
  independently of any PYTHONPATH. Installation via pip remains
  supported. You can skip zkg explicitly via --disable-zkg. See the
  NEWS update for details.

- Establish a "zeek/python" subdirectory under libdir as the common place
  for Python modules in the Zeek distribution. This now separates out
  the Broker Python bindings, ZeekControl, and zkg's Python module.

- Add configure flags to allow customizing this Python folder, in
  three ways: --python-dir, --python-prefix, and --python-home. These
  differ in the logic they automatically add to the path, and build on
  the logic already used in Broker.

- Include a (comented-out) @load for zkg's packages folder in
  local.zeek.

- Bump zeekctl to move to this new location.

- Bump doc to include installation instructions

- Update NEWS accordingly.

configure

@@ -43,6 +43,12 @@ Usage: $0 [OPTION]... [VAR=VALUE]...
     --libdir=PATH          installation directory for library files [PREFIX/lib]
     --conf-files-dir=PATH  config files installation directory [PREFIX/etc]
     --mandir=PATH          installation path for man pages [PREFIX/share/man]
+    --python-dir=PATH      explicit installation directory for Python modules
+    --python-prefix=PATH   versioned installation directory for Python modules,
+                           like setup.py install --prefix
+                           [PATH/lib/python<Python Version>/site-packages]
+    --python-home=PATH     installation path for Python modules, like setup.py's
+                           install --home [PATH/lib/python]
 
   Optional Features:
     --enable-debug         compile in debugging mode (like --build-type=Debug)
@@ -61,6 +67,7 @@ Usage: $0 [OPTION]... [VAR=VALUE]...
     --disable-archiver     don't build or install zeek-archiver tool
     --disable-python       don't try to build python bindings for Broker
     --disable-broker-tests don't try to build Broker unit tests
+    --disable-zkg          don't install zkg
 
   Required Packages in Non-Standard Locations:
     --with-openssl=PATH    path to OpenSSL install root
@@ -157,6 +164,7 @@ append_cache_entry BUILD_SHARED_LIBS    BOOL   true
 append_cache_entry INSTALL_AUX_TOOLS    BOOL   true
 append_cache_entry INSTALL_ZEEK_ARCHIVER BOOL  true
 append_cache_entry INSTALL_ZEEKCTL      BOOL   true
+append_cache_entry INSTALL_ZKG          BOOL   true
 append_cache_entry CPACK_SOURCE_IGNORE_FILES STRING
 append_cache_entry ENABLE_MOBILE_IPV6   BOOL   false
 append_cache_entry ZEEK_SANITIZERS      STRING ""
@@ -201,11 +209,20 @@ while [ $# -ne 0 ]; do
         --prefix=*)
             prefix=$optarg
             append_cache_entry CMAKE_INSTALL_PREFIX PATH   $optarg
-            append_cache_entry ZEEK_ROOT_DIR        PATH   $optarg
+            append_cache_entry ZEEK_ROOT_DIR PATH   $optarg
             ;;
         --libdir=*)
             append_cache_entry CMAKE_INSTALL_LIBDIR PATH   $optarg
             ;;
+        --python-dir=*)
+            append_cache_entry ZEEK_PYTHON_DIR PATH   $optarg
+            ;;
+        --python-prefix=*)
+            append_cache_entry ZEEK_PYTHON_PREFIX PATH   $optarg
+            ;;
+        --python-home=*)
+            append_cache_entry ZEEK_PYTHON_HOME PATH   $optarg
+            ;;
         --scriptdir=*)
             append_cache_entry ZEEK_SCRIPT_INSTALL_PATH STRING $optarg
             user_set_scriptdir="true"
@@ -277,6 +294,9 @@ while [ $# -ne 0 ]; do
             append_cache_entry BROKER_DISABLE_TESTS        BOOL true
             append_cache_entry BROKER_DISABLE_DOC_EXAMPLES BOOL true
             ;;
+        --disable-zkg)
+            append_cache_entry INSTALL_ZKG BOOL false
+            ;;
         --with-openssl=*)
             append_cache_entry OPENSSL_ROOT_DIR PATH $optarg
             ;;