Add missing "O_TRUNC" for writing files.

Actually, it is possible to go one step further and unlink the file
if it exists.  That way, if anyone has hard links to the file, their
"copy" will not be overwritten.  Not sure if it's up to the library
to make that decision, though.
Add vtkDICOMValue methods that use count arguments.
Use size_t instead of unsigned int as interface type.

Using unsinged int is likely to cause many many compiler warnings,
because unsigned int is smaller than size_t and ptrdiff_t on 64 bit
systems.
Clean up vtkDICOMValue code, avoid unnecessary calcs.
Remove deprecated vtkDICOMValue methods from code.
Add another 8-bit field in vtkDICOMValue for overflow.

This is specifically for values that use delimiters instead of fixed
lengths, such as OB or UN VRs that store encapsulated data.  By adding
an 8-bit overflow byte for the 32-bit NumberOfValues field, it becomes
possible to store encapsulated data with sizes up to 2^40 bytes (1TB).
Values that use fixed lengths are still limited to 2^32-2 bytes (4GB)
as per the standard.
Use unsigned char as the basic "byte" type.

This will help to improve security, because the signedness of unsigned
char is always "unsigned", while the signedness of "char" depends on the
compiler.