Since bitbake switched back to the fork instead of the exec model,
it no longer used bitbake-runtask and the code has suffered some bitrot.
bitbake-runtask is a useful tool for excuting the task without
the scheduler of bitbake, so that the external tool can invoke it
easily. It also provides a useful example of how to invoke exec_task()
with low overhead without a lot of the bitbake threading/UI overhead.

Significant changes:

* This patch changes the argument order so that the commonly used
  and mandatory arguments come first.

* The taskhash file and dryrun options are now optional

* It now uses the bitbake logging mechanisms to provide processed
  logging output to the console.

* The process handling to do with stdout/stderr redirection
  are removed since they're no longer required.

[YOCTO #1229]

RP: Logging updates to the patch based on Roberts original patch
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>