Allow passing “NULL” as the device node in umockdev_testbed_load_script

This requires storing the name of the device node umockdev-record is recording
from. This now happens in a "d 0 /dev/path" line at the top of the *.script
Also document that umockdev_testbed_load_script(testbed, NULL, scriptfile) is valid
Merge pull request #38 from RAOF/master

Allow passing “NULL” as the device node in umockdev_testbed_load_script