Fix create_temp_name to replace any '/' found in the hostname with '?'.

Fix create_temp_name to replace any '/' found in the hostname with '?'.

There's a possibility someone will use the '/' in the hostname. Since we
generate a temporary file name (path) including the hostname, any '/' would
be ambiguous.

We can always set such hostname using 'sethostname' from unistd.h. But the
'hostname' command already includes the check and removes the '/' char.
However, some old versions still allow that.
See: https://bugzilla.redhat.com/show_bug.cgi?id=711445.

Since this is only a temporary name and the possibility of this error is
quite negligible, we don't need any complex escape sequence here, just a
simple char replace.