msvcrt: SEEK_* macros from GLIBC should be replaced with safer MSVCRT_SEEK_* macros

[Carlo Bramini]

In dlls/msvcrt/file.c, MSVCRT uses the macros SEEK_SET/SEEK_CUR/SEEK_END from 
GLIBC.

That code also assumes that:
SEEK_SET = FILE_BEGIN
SEEK_CUR = FILE_CURRENT
SEEK_END = FILE_END

It would be better, for providing better portability since the real values of 
the various SEEK_* macros is unknown, to declare some MSVCRT_SEEK_* macros for 
this purpose, as it has been already done for others.

So, according to the above assumptions, the values to be assigned to the new 
MSVCRT_SEEK_* macros have been taken from here:

https://msdn.microsoft.com/en-us/library/windows/desktop/aa365541%28v=vs.85%29.
aspx

Attached patch implements this change (although there are not real changes in 
the code itself).

Sincerely,

Carlo Bramini.

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