stats: fix memory leakage when adding / deleting interfaces

stats: fix memory leakage when adding / deleting interfaces

This fixes two leaks in registering errors in the stats segment.
- The error name created by vlib_register_errors() was not freed.
- Duplicate error names (when interface readded) was added to the vector.

This fix also adds memory usage statistics for the statistics segment
as /mem/statseg/{used, total}

Change-Id: Ife98d5fc5baef5bdae426a5a1eef428af2b9ab8a
Type: fix
Signed-off-by: Ole Troan <ot@cisco.com>