gioerror: map some more values to G_IO_ERROR_NOT_SUPPORTED

Map EPROTONOSUPPORT, ESOCKTNOSUPPORT, EPFNOSUPPORT and EAFNOSUPPORT to
G_IO_ERROR_NOT_SUPPORTED in g_io_error_from_errno(). (GSocket's
socket_io_error_from_errno() already did this with the corresponding
Winsock errors.)

Also map EOPNOTSUPP, which on Linux is the same as ENOTSUP, but may
not be on other platforms.

Also, rewrite the EAGAIN/EWOULDBLOCK section to use the simpler idiom
used by EEXIST/ENOTEMPTY and (now) ENOTSUP/EOPNOTSUPP.
gio/tests/socket: Update to check the error code

Only treat IPv6 as unsupported if the attempt to create a socket
returns G_IO_ERROR_NOT_SUPPORTED: any other error is a bug.
gio: move Winsock error mapping to g_io_error_from_win32_error()

Rather than having special code in gsocket.c, handle Winsock errors
along with other Win32 errors in gioerror.c

Also, reference g_win32_error_message() from the
g_io_error_from_win32_error() docs, and update the
g_win32_error_message() docs to clarify that it works with Winsock
error codes too.