Build a minimized Nix with MinGW

At this point many features are stripped out, but this works:

- Can run libnix{util,store,expr} unit tests
- Can run some Nix commands

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Brian McKenna <brian@brianmckenna.org>

src/libstore/unix/build/child.cc

@@ -0,0 +1,37 @@
+#include "child.hh"
+#include "current-process.hh"
+#include "logging.hh"
+
+#include <fcntl.h>
+#include <unistd.h>
+
+namespace nix {
+
+void commonChildInit()
+{
+    logger = makeSimpleLogger();
+
+    const static std::string pathNullDevice = "/dev/null";
+    restoreProcessContext(false);
+
+    /* Put the child in a separate session (and thus a separate
+       process group) so that it has no controlling terminal (meaning
+       that e.g. ssh cannot open /dev/tty) and it doesn't receive
+       terminal signals. */
+    if (setsid() == -1)
+        throw SysError("creating a new session");
+
+    /* Dup stderr to stdout. */
+    if (dup2(STDERR_FILENO, STDOUT_FILENO) == -1)
+        throw SysError("cannot dup stderr into stdout");
+
+    /* Reroute stdin to /dev/null. */
+    int fdDevNull = open(pathNullDevice.c_str(), O_RDWR);
+    if (fdDevNull == -1)
+        throw SysError("cannot open '%1%'", pathNullDevice);
+    if (dup2(fdDevNull, STDIN_FILENO) == -1)
+        throw SysError("cannot dup null device into stdin");
+    close(fdDevNull);
+}
+
+}