link-parser: Simplify editline interface calls
link-parser: Simplify editline interface calls

Also el_set() can be used with mb_prompt (instead of el_wset() with
wc_prompt like now), but el_wset() allows to use non-Ascii characters in
the prompt if ever desired.

opencog/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,ampli/link-grammar,linas/link-grammar,linas/link-grammar,linas/link-grammar,ampli/link-grammar,linas/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,opencog/link-grammar,ampli/link-grammar,linas/link-grammar,ampli/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar