credential-cache: check for windows specific errors

credential-cache: check for windows specific errors

Connect and reset errors aren't what will be expected by POSIX but
are instead compatible with the ones used by WinSock.

To avoid any possibility of confusion with other systems, checks
for disconnection and availability had been abstracted into helper
functions that are platform specific.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>