core: Minor cleanup of ast_el_read_char.

* Define CHAR_T_LIBEDIT and CHAR_TO_LIBEDIT based on
  HAVE_LIBEDIT_IS_UNICODE.  This avoids needing to repeatedly use
  conditional blocks, eliminates having multiple function prototypes.
* Remove parenthesis from return values.
* Add missing code block brackets {}.
* Reduce use of 'else' conditional statements where possible.

Change-Id: I4315328ebea2f62641faf6881de2ac20a9f9d08e