Memcheck reports an error in libunwind exception raising.

[stable-1.6] Ubsan fixes

Origianl implementation resulted in the following array_iterator_noconv_0_test
failure under UBSAN:

"runtime error: applying non-zero offset 4 to null pointer"

Casting unaligned ptr_offset_magic address to pointers is undefined
behavior for types which require alignemnt > 1. It results in following
error under UBSAN for ptr_0_none test:

"runtime error: upcast of misaligned address 0x0000deadbeef for type 'C',
which requires 8 byte alignment"

Also, add test for offset calculation for objects with different
alignment requirements.

Merge back stable-1.5 to stable-1.6

add "rebuild" param to utils' script, to force dockers rebuilding

it makes more sense to rebuild dockers all the time on stable branches,
because we don't have to count on DockerHub's policy to keep our
docker images on their servers.

docker: make build- and push-image scripts more verbose

Merge stable-1.5 into stable-1.6

otherwise it will still use found (most likely, newer) clang-format.

Update doxygen settings

bump docker images versions

and set clang-format to fixed version (6.0).

fixes #854

Merge stable 1.5 into stable 1.6

contigous_iterator: make operator[] take signed integral instead

of unsigned.

The reason is that Windows cannot decide for a call like this:

`it[0] = 10`

whether to use:
* operator[](unsigned) -- one conversion from signed to unsigned
* operator T*() and then build in operator[](signed) -- one conversion

Merge stable-1.5 into stable-1.6

fix dir creation in auto docs update

doc: enable auto gh-pages generation in docker script

Create separate Docker images for stable-1.6 branch

tests: remove unnecessary #include <libpmemobj.h>

Merge changes from stable-1.5