Add tempfile function.
Use tempfile in place of tempnam.

To avoid new GCC warnings about tempnam:
ncat_connect.c:789: warning: the use of `tempnam' is dangerous, better use `mkstemp'

Doing things this way has the same race condition as tempnam did,
because we are unlinking the file before binding it. (The race window is
smaller now.) The file must not exist before binding the Unix socket, or
else you get an "address already in use" error. Unlinking before binding
is the same thing that netcat-openbsd does. See this earlier thread:
http://seclists.org/nmap-dev/2012/q4/336.