Fix output redirection of exec_prerun/_postrun

The previous version used the bashism "&>" to redirect stdout and stderr of the prerun/postrun commands to an output file. Since system() executes the command using "/bin/sh sh -c …". If /bin/sh is not bash, this wont work.
Use the "posixly-correct" way of redirecting stdout to the output file and then redirect stderr to stdout.

Signed-off-by: Hannes Weisbach <hannes.weisbach@gmail.com>