Replace loading of ldap_start_tls_sA() by direct function call

Replace loading of ldap_start_tls_sA() by direct function call

This change impacts the backend-side code in charge of starting a LDAP
TLS session.  It is a bit sad that it is not possible to unify the WIN32
and non-WIN32 code paths, but the different number of arguments for both
discard this possibility.

This is similar to 47bd0b3, where this replaces the last function
loading that seems worth it, any others being either environment or
version-dependent.

Reported-by: Thomas Munro
Reviewed-by: Thomas Munro
Discussion: http://postgr.es/m/Yx0rxpNgDh8tN4XA@paquier.xyz