include/c: add get_hostname_max() inline function

include/c: add get_hostname_max() inline function

Using sysconf(_SC_HOST_NAME_MAX) does not work everywhere.  At such even
header values for hostname length should be preferred, and if nothing
else works guess the maxium being 64 bytes.

Based-on-code-by: Karel Zak <kzak@redhat.com>
CC: Mike Frysinger <vapier@gentoo.org>
References: http://www.spinics.net/lists/util-linux-ng/msg07050.html
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>