Sync gethostname() with FreeBSD:

Sync gethostname() with FreeBSD:

* Change gethostname() to set errno to ENAMETOOLONG instead of ENOMEM
when the buffer is not long enough to hold the current host name.
POSIX does not standardize error returns for gethostname(), so it
doesn't matter which one we use, but ENAMETOOLONG is at least a little
more intuitive.

* Update prototype to match SuS (int->size_t).