fix a bunch of MSVC compiler warnings

lib/asyn.h

lib/connect.c

lib/connect.h

lib/content_encoding.h

lib/cookie.h

lib/curl_ntlm.c

lib/curl_ntlm.h

lib/curl_ntlm_msgs.c

lib/curl_ntlm_wb.c

lib/cyassl.h

lib/ftp.c

lib/hostip6.c

lib/http_digest.h

lib/imap.c

lib/krb5.c

lib/ldap.c

lib/memdebug.c

lib/memdebug.h

lib/non-ascii.h

lib/nss.c