[Fortran, Patch] Implement IMPLICIT NONE

[Tobias Burnus]

I don't want to implement Fortran 90's implicit none, which is of course 
already supported. However, I would like to implement as vendor extension:

   IMPLICIT NONE (external)

which forces at that least an "external" or "procedure" is used or an 
explicit interface available, if one tries to invoke a procedure. 
Background for the change is my recent bug:
    internal co_broadcast
    call co_broadcasr(...)

While I have implemented is as vendor extension (-std=gnu), of course, 
the syntax doesn't come out of the blue but is in the current Fortran 
2015 draft (14-007r2):

R563 implicit-stmt  is  IMPLICIT implicit-spec-list
                     or  IMPLICIT NONE [ ( [ implicit-none-spec-list ] ) ]
R566 implicit-none-spec  is  EXTERNAL
                          or  TYPE


Build and regtested on x86-64-gnu-linux.
OK for the trunk?

Tobias

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