npfs: opening a pipe without listening server must return STATUS_OBJECT_PATH_NOT_FOUND under some circumstances

r57419 changed NpfsCreate to return STATUS_OBJECT_NAME_NOT_FOUND if no listening server is available. This benefits kernel32_winetest:pipe, but causes a new failure in ntdll_winetest:om.
The previous status STATUS_OBJECT_PATH_NOT_FOUND apparently has to be returned under some conditions as well (presumably involving sub-directories, which om tests). This needs more detailed tests than Wine provides.

Testman comparison: http://reactos.org/testman/compare.php?ids=13733,13735