Cactus: handle error in gethostname

This works around deficiencies of the gethostname call, namely that it does not return an error of the supplied buffer is too small and that it may not include a NUL byte at the end of the buffer if the buffer was too small and the returned name was truncated.

This should not happen (often) in practise since glibc already has a similar workaround in place, but may happen eg on a Mac if they indeed use a non glibc C library.