powerups: fix #2857 and move g_powerups cvar out of server/items/items.qc

powerups: fix #2857 and move g_powerups cvar out of server/items/items.qc

This sets ITEM_FLAG_NORMAL initially and adds ITEM_FLAG_MUTATORBLOCKED
about as late as possible: must be done after mapinfo settemps are
applied, see b95beb262077d8f733bbee7cb0d5f4c05b7b3e2f
and before StartItem() calls have_pickup_item().

Moves a little more powerup-specific code out of common code.

Removes some default powerup balance settings which don't need to be in QC.

Clarifies var naming inside powerup m_iteminit() funcs.