Fix some warnings for size_t, DWORD and int conversions.

Fix some warnings for size_t, DWORD and int conversions.

These cases are all in the sp_[non]blocking_{read,write} functions.

On MSVC, these conversions would generate warnings such as:
warning C4267: '=': conversion from 'size_t' to 'DWORD', possible loss of data

The warnings are genuine. There are some places where overflow is technically
possible, due to our use of size_t for sizes in function parameters (unsigned
64-bit on Windows x64), but an enum for return values (typically signed int
and 32-bit, but not guaranteed to be so by the standards), plus the Win32 API
usage of DWORD (unsigned 32-bit) for sizes in ReadFile/WriteFile.

However, overflow in practice would require reading/writing more than 2GB
over a serial port in a single call and is therefore unlikely to be a
real-world concern. I have therefore not tried to catch those cases - but the
places it is possible do now have explicit casts to the smaller types so that
they are more obvious.

We could document and test for a maximum read/write size of INT_MAX, but that
would still depend on the storage of 'enum sp_return' being at least a signed
int, which as I understand it the C standard does not require.

To be absolutely correct we would need a different API where sp_return
was only used for result codes, and the read/write functions took a
pointer to size_t for result sizes.