Please add support for MidnightBSD

[Hung Nguyen Gia <gh_origin%zohomail.com@localhost>]

My experience with the system so far is it's almost identical to FreeBSD. So we could reuse the make files of FreeBSD for it. I have no idea how to fill the variables in mk/bsd.prefs.mk as the guide here said: https://www.netbsd.org/docs/pkgsrc/pkgsrc.html#porting.opsys

So the only thing I could do is provide you the values that I think could be used to determine these variables:

gh@gh:/usr/home/gh $ uname -s
MidnightBSD
gh@gh:/usr/home/gh $ uname -r
2.0.3
gh@gh:/usr/home/gh $ uname -a
MidnightBSD gh 2.0.3 MidnightBSD 2.0.3 #10 59f5fec7fe(stable/2.0)-dirty: Thu Dec 24 17:41:42 EST 2020     root@skunkworks:/usr/obj/usr/src/sys/GENERIC  amd64
gh@gh:/usr/home/gh $ clang -v
MidnightBSD clang version 8.0.1 (tags/RELEASE_801/final 366581) (based on LLVM 8.0.1)
Target: x86_64-unknown-freebsd11.4
Thread model: posix
InstalledDir: /usr/bin

Hope you could use them to add support for MidnightBSD. Thanks.