Change HAVE_POPEN to USE_POPEN

PS target doesn't support it, so the option should be renamed.

modules/exploits/unix/fileformat/imagemagick_delegate.rb

@@ -25,8 +25,8 @@ class MetasploitModule < Msf::Exploit
         (discovered by taviso) to achieve RCE in the Ghostscript delegate.
         Ghostscript versions 9.18 and later are affected.
 
-        If ImageMagick supports popen(), a |-prefixed command will be used for
-        the exploit. No delegates are involved in this exploitation.
+        If USE_POPEN is set to true, a |-prefixed command will be used for the
+        exploit. No delegates are involved in this exploitation.
       },
       'Author'          => [
         'stewie',            # Vulnerability discovery
@@ -72,7 +72,7 @@ class MetasploitModule < Msf::Exploit
 
     register_options([
       OptString.new('FILENAME', [true, 'Output file', 'msf.png']),
-      OptBool.new('HAVE_POPEN', [false, 'popen() support', true])
+      OptBool.new('USE_POPEN',  [false, 'Use popen() vector', true])
     ])
   end
 
@@ -87,7 +87,7 @@ class MetasploitModule < Msf::Exploit
   end
 
   def template
-    if datastore['HAVE_POPEN']
+    if datastore['USE_POPEN']
       t = 'popen'
     else
       t = 'delegate'