* read.h (SCM_N_READ_OPTIONS): increase SCM_N_READ_OPTIONS to 4.

	* read.h (SCM_N_READ_OPTIONS): increase SCM_N_READ_OPTIONS to 4.
	(SCM_KEYWORD_STYLE): defined.

	* read.c (scm_read_opts): add a keywords option.  This isn't a
	boolean option, in case someone wants to add support for DSSSL
	keywords too.
	Setup scm_keyword_prefix symbol.
	(scm_lreadr): Only process keywords if SCM_KEYWORD_STYLE is
	set to 'prefix.
*	I've left keyword support disabled by default, since it doesn't
	seem to break the module system and it gives R4RS standard behaviour.
	It can be reactivated with (read-set! keywords 'prefix).