bug#1863: Improved grep prompt

[jidanni]

Regarding M-x grep, here are the current and improved prompts:
Run grep (like this): grep -nH -e
Run grep (like this): grep -nH -e [Cursor goes here] * <--and an asterisk here. 
I.e., @=cursor:
Run grep (like this): grep -nH -e @ *
If you use my improved prompt suggestion above, no longer will
everybody forget to add a filename, and leave thinking "no matches found".