If a timeout is set, perform the SSL Handshake using non-blocking IO.  This way
we can timeout if SSL Handshake gets stuck for whatever reason.

This code is currently hidden behind #ifdefs (LDAP_USE_NON_BLOCKING_TLS) and
disabled by default as there seem to be some problems using NON-blocking
I/O during the TLS Handshake when linking against NSS (either a bug in NSS
itself of in tls_m.c, see discussion on -devel)

This patch adds an additional parameter to ldap_int_poll() in order to indicate
if we're waiting in order to perform a read or write operation.

error, not LBER_ERROR.

and Rafal Szczeniak <mimir@samba.org>, with minor cleanups

don't assert(0); apparently, other responses may result from a simple bind (but log them, at least; ITS#4924)

	- after connect(2), if async the connection is in "connecting" state
	- the first time a request is sent, "connecting" conns are polled
		- in case of success, the request is sent
		- in case still connecting, LDAP_X_CONNECTING is returned;
		  clients are expected to retry later
	- the "async" behavior must be explicitly enabled by setting
	  the LDAP_OPT_CONNECT_ASYNC option

"local" connections need work

(with notable exceptions: ignore tests for EINTR which winsock never sets)