Filter/C++: Fix potential memory leak.

Filter/C++: Fix potential memory leak.

dlclose() should not be called if the target-library's contents may
be refered later.

dlclose() should be called at the exit of a pipeline to avoid
memory leaks of user applications.

Thus, we need to call dlclose for all handles at the ensured exit
point.

They are named with underbar (_) because register is a keyword.

Changes in v2:
- Added lock for handles
- Don't call dlclose at exit context if it's GLIBC 2.23

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>