Socket handle should use "unsigned" types on Windows

[irwir]

Summary

Networking implementation in the library follows POSIX standards, but the same code is used on Windows too.

mbedtls/include/mbedtls/net_sockets.h

Line 90 in f19a08f

int fd;

There are no standard options to change this declaration, though Windows declares socket as unsigned pointer-sized integer.
The current code has two negative properties:

• Implicit type casts between 32-bit/64-bit unsigned integers and 32-bit signed integers - with potential loss of data.
• Check for invalid socket is not quite correct, because there is one specific value on Windows, not any negative number.

System information

Mbed TLS version (number or commit id): any
Operating system and version: Windows
Configuration (if not default, please attach mbedtls_config.h):
Compiler and options (if you used a pre-built binary, please indicate how you obtained it):
Additional environment information:

Expected behavior

Using native socket type and error checks on Windows.

Actual behavior

Mbedtls always uses 32-bit signed integer for sockets; any negative value means invalid socket.
Signed/unsigned mismatch and truncation/expansion of values are commonly diagnosed by compilers and static analysers.