[krbdev.mit.edu #1794] don't use mktemp

[Public Submitter via RT]

GCC 3.2.2 (default with RedHat 9) puts this out every time I compile.

I'm using 1.2.7, and would've upgraded to get rid of this message,
except I see it still exists in 1.3.1.

/usr/kerberos/lib/libkrb5.a: In function `krb5_fcc_generate_new':
: the use of `mktemp' is dangerous, better use `mkstemp'