* read.c (potable): Add "mri" and ".mri".

	* read.c (potable): Add "mri" and ".mri".
	(s_mri): New function.
	* read.h (s_mri): Declare.
	* app.c (scrub_m68k_mri): New static variable.
	(mri_pseudo): New static variable.
	(do_scrub_begin): Add m68k_mri parameter.  Use it rather than
	flag_m68k_mri.  Initialize scrub_m68k_mri.
	(mri_state, mri_last_ch): New static variables.
	(struct app_save): Add scrub_m68k_mri, mri_state, and mri_last_ch
	fields.
	(app_push): Save new fields.
	(app_pop): Restore new fields.
	(do_scrub_chars): Check scrub_m68k_mri rather than flag_mri_mri.
	If TC_M68K, use a trivial state machine to look for occurrences of
	the .mri pseudo-op, and change the mode appropriately.
	* as.h (do_scrub_begin): Update prototype.
	* input-scrub.c (input_scrub_begin): Pass flag_m68k_mri to
	do_scrub_begin.
	* config/tc-m68k.c (reg_prefix_optional_seen): New static
	variable.
	(m68k_mri_mode_change): New function.
	(md_parse_option): Set reg_prefix_optional_seen.
	* config/tc-m68k.h (m68k_mri_mode_change): Declare.
	(MRI_MODE_CHANGE): Define.
	* doc/as.texinfo: Document .mri pseudo-op.