Add support for DragonflyBSD and NetBSD.

This merges all the bsd (including darwin) support into terminal_bsd.go

Cross-compiling works for all of them in that they all compile, but I've only
actually tested the binary on dragonflybsd.  I don't see why this wouldn't
work on the others since I don't think they'd compile if they didn't
support TIOCGETA and Termios, but just a heads up.

terminal_bsd.go

@@ -0,0 +1,9 @@
+// +build darwin freebsd openbsd netbsd dragonfly
+
+package logrus
+
+import "syscall"
+
+const ioctlReadTermios = syscall.TIOCGETA
+
+type Termios syscall.Termios