Description of problem:
If using a custom SSL context through the LDAP_OPT_X_TLS_NEWCTX LDAP option, openldap will call nss_ShutdownContext on each unbind.  When the last connection is unbound, NSS will call nss_Shutdown to unload the entire NSS library.  Once this happens, if using PEM files, the next unbind will result in SEC_BUSY with the following text:  TLS: could not shutdown NSS - error -8053:NSS could not shutdown. Objects are still in use..

This happens because the tlsm_nss_shutdown_cb is only registered once, and nss_Shutdown will unregister it.  tlsm_nss_shutdown_cb is responsible for unloading the nss PEM module.  Since the shutdown function is no longer called, the PEM module is still loaded, and the NSS module refuses to shut down correctly.  Eventually this causes many other strange issues such as complaints that no slots are available for the certificates that it reads.

Version-Release number of selected component (if applicable):
openldap-2.4.23-34.el6_5.1.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Create a LDAPS connection using a custom context by calling ldap_set_option with LDAP_OPT_X_TLS_NEWCTX.  This also requires manually setting the LDAP_OPT_X_TLS_CACERTDIR or LDAP_OPT_X_TLS_CACERTFILE, LDAP_OPT_X_TLS_CERTFILE, and LDAP_OPT_X_TLS_KEYFILE, as a minimum.
2. Bind to an LDAPS server, then unbind
3. Bind again, then unbind.

Actual results:
TLS: could not shutdown NSS - error -8053:NSS could not shutdown. Objects are still in use..  and other strange behavior

Expected results:
Continued operation in binding and unbinding.