bugfix for #3881. "Escaped html in error message"

The reason for the bug is the post request for "import.php" by *_import.php files. So, in this case after logging in, the target variable in the GET REQUEST was always set to import.php and thus PMA was redirected to import.php. But import the php used two parameters import_type and format which when checked in the checkparameters function resulted in fatal error.

The changes that i made were, before redirecting if the request was for import.php in the code of index.php, if true i used session variable to state that i am doing some redirection(this variable can be pretty helpful if in future we have some more legacy submissions like for import.php) and in session variable i check it if it's true or not,

If true then just redirect the code to the last url where we were(which is also stored in session, could have used $GLOBALS but the seems to be reloaded by the common.inc.php, so used sessions)