Update usage information for genPOI

Usage text update to include --quiet and reflect whether script is
called independently or via the --genpoi flag for overviewer.py

genPOI.py

@@ -17,6 +17,7 @@ markers.js holds a list of which markerSets are attached to each tileSet
 import os
 import logging
 import json
+import sys
 from optparse import OptionParser
 
 from overviewer_core import logger
@@ -41,8 +42,13 @@ def handleSigns(rset, outputdir, render, rname):
 
 
 def main():
-    helptext = """genPOI
-    %prog --config=<config file>"""
+
+    if os.path.basename(sys.argv[0]) == """genPOI.py""":
+        helptext = """genPOI.py
+            %prog --config=<config file> [--quiet]"""
+    else:
+        helptext = """genPOI
+            %prog --genpoi --config=<config file> [--quiet]"""
 
     logger.configure()