space: add xc suffix to public functions that may throw

There are four such functions declared in space.h:

  space_new
  index_find_unique
  index_find_system
  access_check_space

index_find_unique() and access_check_space() are used in the code that
is about to be converted to C so for them add exception-free variants
(without xc suffix).

Preparation for converting space implementation to C.

Needed for #2776