Add secure gethostname() wrapper

Add secure gethostname() wrapper

gethostname() func does not ensure null-terminated string
if hostname is longer than buffer length.
For security, a function s_gethostname() has been added
to ensure that "\0" is added to the end of the buffer.
Previously this had to be handled in each place
of the gethostname() call.

Signed-off-by: Blazej Kucman <blazej.kucman@intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>