In |NCONF_load|, strdup section names rather than adding refs.

The fact that |value_free| expects to free() value->section is
inconsistent with the behavior of |add_string|, which adds a reference
to an existing string.

Along the way, add a |CONF_VALUE_new| method to simplify things a bit.

Change-Id: I438abc80575394e4d8df62a4fe2ff1050e3ba039
Reviewed-on: https://boringssl-review.googlesource.com/5744
Reviewed-by: Adam Langley <agl@google.com>