GC4 issues to fix

[ddeclerck]

This lists issues that came up when merging GC3 patches to GC4.

• [ ] 1. Decide what to do about gcdiff, espacially under MSVC (see here for details)
• [ ] 2. Correctly implement delay-loading under MSVC
• [ ] 3. Possibly drop usage of external cobxref (see here for details)
• [ ] 4. Fix any remaining failed test case (especially MSVC ones)
• [ ] 5. Investigate the need to define READ_WRITE_NEEDS_FLUSH under MacOS ("LINE SEQUENTIAL COMMIT / ROLLBACK" test fails if not defined)
• [ ] 6. Investigate INITIALIZE BY VALUE (currently under #if 0) in field.c:validate_field_value
• [ ] 7. Investigate the TODO in codegen.c:output_initialize
• [ ] 8. Investigate the two CHECKMEs about bdb_close_cursor in fbdb.c:ix_bdb_write_internal
• [ ] 9. Check what we should do about the casts used to remove const on open_mode in fileio.c and others (eg. in cob_open_file)
• [ ] 10. Check if shoudld use strcpy or memcpy with computed max-length for file_open_name in fileio.c:cob_open
• [ ] 11. Check the code for setting last_exception_source (currently under #if 0) in common.c:cob_set_exception
• [ ] 12. Investigate the CHECKMEs in typeck.c:validate_move and in particular why it was necessry to add CB_LITERAL_P (checks to add in parser)