cli: use plain readline() interface with libedit

cli: use plain readline() interface with libedit

Instead of the alternate interface [1].

I spent a bit of time debugging an issue with libedit support
9420423900a2 ("cli: add libedit support") that broke tests/shell.

This is the reproducer:

 # nft -i << EOF
 list ruleset
 EOF

which makes rl_callback_read_char() loop forever on read() as shown by
strace. The rl_line_buffer variable does not accumulate the typed
characters as it should when redirecting the standard input for some
reason.

Given our interactive interface is fairly simple at this stage, switch
to use the readline() interface instead of rl_callback_read_char().

[1] https://docs.freebsd.org/info/readline/readline.info.Alternate_Interface.html

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>