Some patches to make an almost silent "gcc -Wall -W2"
compile on MingW. Mostly replacing '-1' with 'CURL_SOCKET_BAD'.

My only concern is a patch like:
- if (sockfd < 0) {
+ if (sockfd == CURL_SOCKET_BAD) {

But I've yet to hear of a stack returning < -1 on fail.

I added a new temp variable 'readfile_read' in telnet.c because
sizeof(ssize_t) != sizeof(DWORD) on a Win64 box.

--gv