PKG_FEAT, Set special features

rst $08, defw $120f


IN:	B=mask for features to change
	C=feature settings
	E=process number to trace, if setting single process trace
OUT:
	Fc=0, success
	C=current feature settings
	E=process number, if single process being traced
	B=# installed package interrupts
	D=# installed process interrupts

Registers changed after return:
  ......HL/IXIY same
  AFBCDE../.... different

Bit settings (for B & C) are:
	0: Interrupt chain processing
	1: CALL_OZ substitution
	2: Call tracing
	3: Single process tracing (1); global tracing (0)
	(Bit 3 can only be set at the same time as bit 2)

Notes:
	For use by Installer only
	Call tracing is not active on FP_CALL, CALL_PKG or
	OZ_DI/OZ_EI/OZ_BUFF calls; only standard CALL_OZ calls
