Update Fallout 4 load order implementation

[Ortham]

As of the availability of the Survival Mode beta for Fallout 4, the load order mechanism has changed. plugins.txt now lists all installed plugin filenames in load order. Active plugins have their filenames prefixed by an asterisk *, for example:

*Fallout4.esm
ArmorKeywords.esm
Armorsmith Extended.esp
Field Scribe Army - AE Comp.esp
*EasyHacking.esp
*Test_WEAP.esp
*Simple Intersection.esp

In the above, Fallout4.esm, EasyHacking.esp and Test_WEAP.esp are all active, and the other plugins are not active.

This new mechanism obsoletes loadorder.txt, so libloadorder should no longer write that file for Fallout 4. I'll call the new mechanism the "asterisk" load order method (so the constant will be LIBLO_METHOD_ASTERISK).

I'll need to update the test suite to check that data is written correctly for the new method, then update the corresponding code accordingly.

[Ortham]

Done in a81db4c