convert reg_cache_limit to seq_file and proc_create

writing out to user space with seq_printf is fun, but reading in from
userspace, as far as i can see it, is not.

kept but corrected function signature of scif_set_reg_cache_limit as
scif_reg_cache_limit_write and converted scif_get_reg_cache_limit to the
new seq_file way to scif_reg_cache_limit_show.