build, core, internet: MinGW/Msys2 support patches

Changes:

1. Add support to module "private" headers (not listed in *-module.h headers).

2. Add the win32-internet.h header. This header includes winsock2.h, which replaces unix headers such as netinet/in.h, sys/socket.h and others. This header also undefines colliding symbols, since winsock2.h includes way more than needed.

3. Link Windows internet libraries (ws2_32 and wsock32) to the core module (required by fd-reader).

4. Change time rounding from double to long int (32bit on Windows) into long long int.

5. Add Windows section in the manual, including how to setup MinGW64 using a native shell (cmd/powershell) or using the unix-like terminal with a bash shell provided with Msys2. Also includes packaging instructions for Vagrant VM's.

6. Fix int64x64 test suite on Windows by replacing the Valgrind (unsupported) check with 1, to indicate long doubles are 64-bit and require bigger margins of error.