[tex-live] adding support for MidnightBSD in tlmgr

[Karl Berry]

            amd64-unknown-midnightbsd1.2

I added it to platform_name and platform_desc in TLUtils.pm.

Also, in order not to have add every OS in the world to our little list,
I added a check for the envvar TEXLIVE_OS_NAME. If that is set, it is
used as-is without any guessing. (The <cpu> value is still taken from
the config.guess output, though.)

So something like
  env TEXLIVE_OS_NAME=anotherbsd tlmgr ...

should now have (essentially) the same effect as adding "anotherbsd" to
the lists in the code.

    although I did look at TLUtils.pm to see whether the changes might
    be trivial, there is too much magic in perl statements for me to do
    that with any confidence.

Thanks for trying. Although it is true that there are a lot of
surrounding special cases, the main list of OS names is this
straightforward assignment:

   my @OSs = qw(aix cygwin darwin dragonfly freebsd hpux irix
                kfreebsd linux midnightbsd netbsd openbsd solaris);

I just added "midnightbsd" to that list. --thanks, karl.