schannel_recv: return the correct code

Bug: http://curl.haxx.se/bug/view.cgi?id=1462
Reported-by: Tae Hyoung Ahn
VC build: added sspi define for winssl-zlib builds.
Curl_client_write() & al.: chop long data, convert data only once.
smb: fix unused return code warning
synctime.c: fixed user-agent setting.

Some websites meanwhile refuse to reply to requests from ancient
browsers like IE6, therefore I've comment out this setting, but
also fixed the string to now fake IE8 if someone enables it.
cookies: Improved OOM handling in cookies

This fixes the test 506 torture test. The internal cookie API really
ought to be improved to separate cookie parsing errors (which may be
ignored) with OOM errors (which should be fatal).
sws.c: Fixed 'rc' may be used uninitialized warning
ldap: rename variables to comply to curl standards
ldap: check Curl_client_write() return codes

There might be one or two memory leaks left in the error paths.