* Add a new portage.util.writemsg_level() function which uses a "level"

* Add a new portage.util.writemsg_level() function which uses a "level"
  parameter to control whether the message is sent to stdout or stderr.
  When level >= logging.WARNING then the message is sent to stderr,
  otherwise it is sent to stdout. The noiselevel is passed directly to
  writemsg().

* Use writemsg_level() for handling merge messages from dblink and add
  appropriate noiselevel values to each call.

* Add a writemsg_level parameter to portage.env_update() so that dblink
  can pass in dblink._display_merge() as a means to redirect output back
  to the scheduler for appropriate handling when running in the background.

svn path=/main/trunk/; revision=11098