threadsafe get-byte-or-eof, peek-byte-or-eof

threadsafe get-byte-or-eof, peek-byte-or-eof

* libguile/ports.h (scm_get_byte_or_eof_unlocked):
  (scm_peek_byte_or_eof_unlocked): Rename, adding _unlocked.
* libguile/ports.c (scm_get_byte_or_eof, scm_peek_byte_or_eof): Add
  locking implementations.  Adapt callers to use _unlocked variants;
  they will do locking.

* libguile/read.c (read_token, scm_read_semicolon_comment)
  (scm_read_shebang): Use unlocked variants.  We will add locking
  later.