use HOST_NAME_MAX to size buffers for gethostname(2)

use HOST_NAME_MAX to size buffers for gethostname(2)

POSIX limits the length of host names to HOST_NAME_MAX.  Export the
fallback definition from daemon.c and use this constant to make all
buffers used with gethostname(2) big enough for any possible result
and a terminating NUL.

Inspired-by: David Turner <redacted>
Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: David Turner <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>