Merge back changes from stable 1.6

Use libunwind

Change logic for finding font in pmpong example.

Klockwork fixes

Add option to disable hashmap

[1.6] string: fix max_size() return value

string: change sso_capacity so that sizeof(string) is always 32 bytes

(which is minimal possible size and equal to sizeof(vector)) and
change way of sso and large string detection.

common: fix Coverity skips with proper variable

[1.6] Add missing include to v.hpp.

Needed by std::forward_as_tuple.

common: correction of 'equal to' and quotation marks

New members order makes it possible to use union and exploit vector
_size member to store some additional information (e.g. on MSB).

Merge back changes from Stable 1.6

common: sort arguments alphanumerically in Dockerfiles

test: add iterator tests for string

Currently, if font is found during build then LIBPMEMOBJ_CPP_PMPONG_FONT_PATH is
set accordingly. Otherwise user has to specify path using environment variable.

Reason: do not test undefined behaviour

string: separate declaration and definition

[1.6] cmake: require c++11 for cow check

common: update check-licence to the newest one from PMDK

[1.6] Bump pmdk to 1.6 in Docker images

C++ version was incorrectly computed by cmake to c++98 which resulted in compilation error on ubuntu 16.04

test: add tests for string accessors