Attempts several methods of getting the system hostname and
then caches the result. It tries the first available of the C
library's gethostname(), `$Config{aphostname}`, uname(2),
syscall(SYS_gethostname), `hostname`, `uname -n`,
and the file /com/host. If all that fails it croaks.
All NULs, returns, and newlines are removed from the result.