Added optional logging of startx output

Previously all startx output from cdm-xlaunch was sent to /dev/null.
This commit adds a 'startxlog' configuration option which allows this
behaviour to be overridden.
Merge pull request #30 from 12qu/master

Added optional logging of startx output
Exit with return code 2 when dialog selection was canceled

This makes it more practical to use 'cdm && exit' in a tty when you
decide to cancel when you're already in the dialog selection window.
Merge pull request #31 from polyphemus/exitcancel

Exit with return code 2 when dialog selection was canceled