ITS#7428 Use non-blocking IO during SSL Handshake

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.