RFC: turn off word splitting for vars but keep for read

[Aleksey Midenkov]

Since word splitting in vars is used not more frequently than never,
protecting all vars in scripts with double quotes is quite unpleasant
thing. Therefore, I want exactly this behavior always to be in my
scripts:

$ IFS=""
$ f() { echo $1; }; x="a b c"; f $x
a b c

But, IFS influences `read`, which of course benefits from word
splitting. In light of that conflict, I propose two possible
solutions:

1: shopt -s unsplit_vars

which will turn off word splitting for var expansion.

or 2: new IFS2 var that will override IFS for `read` (i.e. `read` will
use IFS2 when IFS is unset).