Fix Windows CHD support

[CatMe0w]

Windows CHD support was broken: WRN libchdr load failed, chd support is disabled err="The parameter is incorrect."

Caused by two separate bugs, fixed here:

1. ar x on the MinGW import library (libchdr.dll.a) extracted a 56-byte COFF import stub and shipped that as libchdr.dll, so the real DLL was never packaged. The actual DLL lives in the build dir on Windows, so it's now taken from there.
2. LoadLibraryEx was called with a relative name, which requires a absolute path. Fixed in load_library_windows.go.

A working Windows artifact is available in my fork for verification: https://github.com/CatMe0w/ps3netsrv-go/releases

[xakep666]

@CatMe0w thank you for fixes.

If you have a large games collection compressed in chd I'm kindly asking to test #43. That MR should improve performance of directory listing if this directory contains a lot of chd's.

UPD: it's unnecessary now. I'll review this pr a bit later

[xakep666]

Thank you! Merging