Replaced sprintf in the unit test.
Defined constants for sizes and used them.
Wrpapped lines where noticed that they are longer than 80.
Added comments to the places where sprintf is still used
but it is safe to use.

Move GNU_SOUCE definition to configure.ac
Change all headers to use config.h.
Move config.h to the top.
Remove errno.h if it is not needed.

Address CLANG issues in the main code.

The doxygen files contained some deprecated values and typos.
This patch cleans them for all interfaces exposed by ding libs.

Use correct macro instead of type casting.

The header structure might contain padding to align data on
the boundary of the word. It is unsafe to always print
the header data as it might be uninitialized.
I do not want to remove it as it might be handy some day
so I prefer to #ifdef it.

This is the issue found by Coverity. If the proprty is NULL
but type is not there is dereferencing of the NULL veriable
in the check.