Allow setting log file from outside the module

In case the bundle current working directory is not writable by
current non-root user, the logging will fail. This change keeps
the same default but allows overriding the logfile name and path
from outside the module.
Also helps in case user doesn't want logging by setting the logfile
to /dev/null

lib/ruote/util/dlog.rb

@@ -30,14 +30,16 @@ module Ruote
   #
   module D
 
+    def self.set_logfile(fn)
+      f = File.open(fn, 'wb')
+      f.sync = true
+      f
+      @@logfile = f
+    end
+
     def self.log(text, h, *args)
 
-      @@log ||=
-        begin
-          f = File.open("dlog_#{$$}.txt", 'wb')
-          f.sync = true
-          f
-        end
+      @@logfile ||= self.set_logfile("dlog_#{$$}.txt")
 
       t = Time.now
       t = "#{t.strftime('%H:%M:%S')}.#{sprintf('%06d', t.usec)}"
@@ -66,8 +68,8 @@ def self.log(text, h, *args)
       m.concat(args)
       m << "\n"
 
-      @@log.print(m.collect(&:to_s).join(' '))
-      @@log.flush # hard
+      @@logfile.print(m.collect(&:to_s).join(' '))
+      @@logfile.flush # hard
     end
   end
 end