Move IFileOS to its own file

Change-Id: Iee65587b5f1177f05463dab88d81b365bb6e9e81
Detect Monodevelop unit test running in MiscUtils.RunningTests

Change-Id: I1b6e566c3cade2ee19d49eb35fd0f6d070e4eaf3
Add IFileOS.CreateDirectory

So can create directories in mock filesystem in unit tests

Change-Id: I89aa6938890c134cddfcc724408409bec5848d93
Make LinkedFiles dir if default setting but missing

Fixes FWNX-1092. See also LT-14491.

Change-Id: I1c5bc0b827033f097c4871329d47452a19ab39cc