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.

scripts/site/local.zeek

@@ -104,3 +104,6 @@ redef digest_salt = "Please change this value.";
 # Uncomment the following line to enable logging of link-layer addresses. Enabling
 # this adds the link-layer address for each connection endpoint to the conn.log file.
 # @load policy/protocols/conn/mac-logging
+
+# Uncomment this to source zkg's package state
+# @load packages