dtlogin: don't try to use /dev/mem to get random data on linux/bsd

dtlogin: don't try to use /dev/mem to get random data on linux/bsd

dtlogin's genauth routines were trying to open and read /dev/mem on
linux and (presumably) bsd systems in order to obtain random data used
in creating an auth key.

This is bad for a variety of reasons.  Newer linux kernels (at least
on 3.2) issue the following warning to the kernel logs:

"Program dtlogin tried to access /dev/mem between 100000->102000."

Now on linux we will use /dev/urandom, and on CSRG_BASED (bsd) systems
we will use /dev/random to obtain some entropy.