PKG_NXT, Find next installed package ID

rst $08, defw $0e0f


IN:	A=last package ID, or 0
OUT(success):
	Fc=0
	A=next installed package ID
OUT(failure):
	Fc=1, no more installed packages

Registers changed after return:
  ..BCDEHL/IXIY same
  AF....../.... different


Notes:
	For use by Installer only
