bug#26205: Unhappy with deprecating GREP_OPTIONS

[Ankeet]

>> Why is GREP_OPTIONS deprecated?   
> The main reason is that it breaks shell scripts. Scripts generally 
> expect 'grep' to behave as specified in POSIX and as implemented via 
> traditional 'grep' implementations. GREP_OPTIONS breaks this.  
> Some shell scripts work around the problem by explicitly unsetting 
> GREP_OPTIONS before they call 'grep'. However, most scripts don't, I 
> expect because their authors are not generally aware of this 
> compatibility issue. So in practice the convenience of GREP_OPTIONS can be 
> more trouble than it's worth.

Here's a simpler fix: grep should test for an interactive shell before parsing 
GREP_OPTIONS. [There are a number of methods outlined in the man 
page](http://www.tldp.org/LDP/abs/html/intandnonint.html). At least two of 
these methods (PS1[1] and test "$($0)” are POSIX compatible if i’m not mistaken.

Interactive shell users can still export GREP_OPTIONS to get a persistent 
configuration. But this way, shell scripts get grep without the variable set, 
and if they really need to, can still use those options by calling “grep 
$GREP_OPTIONS …” explicitly.