[RFC] fseek, an alternative implementation (PR22359)

[Daniel Franke]

While working on the docs last week, I learned, that the FSEEK intrinsic is 
not implemented yet. 

The FSEEK implementation of g77 takes a statement label to jump to, if an 
error occurs. Since there is no other intrinsic that takes a statement label, 
considerable effort would be necessary to implement the framework needed. As 
demand is low (one PR, no dupes, no usage of FSEEK shown by google-code 
search), it might be worthwile to implement the FSEEK intrinsic for 
completeness and replace the statement label by a status flag:

  g77:        CALL FSEEK(unit, offset, whence, *100)
  gfortran:   CALL FSEEK(unit, offset, whence, status)
              IF (status /= 0) goto 100

This is not 100% compability, but reasonable close.

Please find a preliminary patch that implements FSEEK using a status flag 
attached. Of course, this implementation (still) has the same problems with 
casting/assigning of the status flag as the other intrinsics. 

gcc/fortran:
	* intrinsic.c (add_subroutines): Added FSEEK.
	* intrinsic.h (gfc_resolve_fseek_sub, gfc_check_fseek_sub): New.
	* iresolve.c (gfc_resolve_fseek_sub): New.
	* check.c (gfc_check_fseek_sub): New.
	* intrinsic.texi (FSEEK): Updated.

libgfortran:
	* io/intrinsics.c (fseek_i[1248]_sub): New.
	* gfortran.map (fseek_i[1248]_sub): New.

Comments are highly welcome :)

	Daniel

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