cli: Fix ast_el_read_char to work with libedit >= 3.1

Libedit 3.1 is not build with unicode on as a default and so the
prototype for the el_gets callback changed from expecting a char buffer
to accepting a wchar buffer.  If ast_el_read_char isn't changed,
the cli reads garbage from teh terminal.

Added a configure test for (*el_rfunc_t)(EditLine *, wchar_t *) and
updated ast_el_read_char to use the HAVE_ define to detemrine whether
to use char or wchar.

ASTERISK-26592 #close

Change-Id: I9099b46f68e06d0202ff80e53022a2b68b08871a