[Bug-readline] improve default behaviour in inputrc

[William R]

version : readline 6.2

Hello,
there is one command I always have to configure in inputrc: it is about 
the behaviour of the tab key when using completion in terminals. By 
default, we have to press 2 times the tab button to get proposed 
available text if completion does not have a unique solution.

the following change in inputrc solves it :
set show-all-if-unmodified on

This change is very light, and I really do not see what is interesting 
in not putting this option activated by default.

The other default setting is more personnal, but there are many forums 
on discussions on the net about it :
set bell-style visible

Best regards