Simplify the reader's `read_complete_token'.

Simplify the reader's `read_complete_token'.

* libguile/read.c (read_token): Remove unneeded `const' before `size_t'.
  (read_complete_token): Remove `overflow_buffer' parameter; return
  `char *' instead of `int'.  Allocate the overflow buffer with
  `scm_gc_malloc_pointerless' instead of `scm_malloc'.  Return either
  the overflow buffer or BUFFER.
  (scm_read_number, scm_read_mixed_case_symbol,
  scm_read_number_and_radix): Rename `buffer' to `local_buffer', and
  `overflow_buffer' to `buffer'.  Remove `overflow'.  Adjust code to new
  `read_complete_token'.