Rename our version to `UE_phy_stub_standalone_pnf_task`

Going wild..

Instead of looping over UEs and all DL config Req/HI DCI0, loop first
over all DL config Req/HI DCI0, and for each, match UE.  Lowers CPU
traffic for high traffic situations.

UE_list_t is now used as a separate list. It can be e.g. used in the
scheduler to mark groups of users. The original UE_list_t is renamed to
UE_info_t to reflect that it is a container for information about UEs.