refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/errno_unix.go

@@ -0,0 +1,37 @@
+// Code generated by go generate; DO NOT EDIT.
+// Generated: 2021-09-28 12:05:19.763535 +0200 CEST m=+0.000187668
+
+package netxlite
+
+import "golang.org/x/sys/unix"
+
+const (
+	ECANCELED       = unix.ECANCELED
+	ECONNREFUSED    = unix.ECONNREFUSED
+	ECONNRESET      = unix.ECONNRESET
+	EHOSTUNREACH    = unix.EHOSTUNREACH
+	ETIMEDOUT       = unix.ETIMEDOUT
+	EAFNOSUPPORT    = unix.EAFNOSUPPORT
+	EADDRINUSE      = unix.EADDRINUSE
+	EADDRNOTAVAIL   = unix.EADDRNOTAVAIL
+	EISCONN         = unix.EISCONN
+	EFAULT          = unix.EFAULT
+	EBADF           = unix.EBADF
+	ECONNABORTED    = unix.ECONNABORTED
+	EALREADY        = unix.EALREADY
+	EDESTADDRREQ    = unix.EDESTADDRREQ
+	EINTR           = unix.EINTR
+	EINVAL          = unix.EINVAL
+	EMSGSIZE        = unix.EMSGSIZE
+	ENETDOWN        = unix.ENETDOWN
+	ENETRESET       = unix.ENETRESET
+	ENETUNREACH     = unix.ENETUNREACH
+	ENOBUFS         = unix.ENOBUFS
+	ENOPROTOOPT     = unix.ENOPROTOOPT
+	ENOTSOCK        = unix.ENOTSOCK
+	ENOTCONN        = unix.ENOTCONN
+	EWOULDBLOCK     = unix.EWOULDBLOCK
+	EACCES          = unix.EACCES
+	EPROTONOSUPPORT = unix.EPROTONOSUPPORT
+	EPROTOTYPE      = unix.EPROTOTYPE
+)