Fix reader options for R6RS `get-datum'

[Andreas Rottmann]

This patch series addresses the problem that `get-datum' is using the
global reader options, even for those options that have to have fixed
values to make the reader behave in an R6RS-compatible way.

* [PATCH 1/3] Split r6rs-ports.c according to module boundaries

  Needed by the last patch, since we want to add a private helper
  function for use in `(rnrs io ports)' there.

* [PATCH 2/3] Add internal API to specify reader options at reader

  This provides the basic mechanism, a.k.a. `scm_i_read()'.

* [PATCH 3/3] Make `get-datum' conform more closely to R6RS semantics
  
  And finally, this makes use of `scm_i_read()' to implement
  `get-datum'.