[gfortran] Fix PR23661, allow PRINT with format variable again

[Tobias.Schlueter@Physik.Uni-Muenchen.DE]

PR23661 happened because of the new code to handle namelists in PRINT
statements.  Unfortunately, this code had a shortcoming in that if a symbol
which is not a namelist is encountered, the parser would not back up correctly,
leaving the following matchers with a nonsensical line which they rightfully
refused.

The attached patch fixes this and also adds a check that there's a blank between
a PRINT statement and the namelist name in free form source code.

Bubblestrapped and tested, a number of testcases is also included.  OK for
mainline and 4.0?

- Tobi