Fix TLS support config.

USE_TLS enabled STARTTLS support, which is not wanted for connecting
to ldaps port.

However, disabling USE_TLS prevented configurating for the TLS_CA
option, meaning it was not possible to configure the TLS_CA
unless using STARTTLS.

TLA_CA is now always configured, and USE_TLS option renamed to
START_TLS to more accurately reflect what it does.

Also implement new option, REQUIRE_TLS, to require a TLS connection, and
fail if one is not available.

Change-Id: I8097e6f64eb7b0a538876bb6bb349c5781f724b4