nix-env: Add regular expression support in selectors

So you can now do things like:

  $ nix-env -qa '.*zip.*'
  $ nix-env -qa '.*(firefox|chromium).*'

src/libutil/regex.hh

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "types.hh"
+
+#include <sys/types.h>
+#include <regex.h>
+
+namespace nix {
+
+class Regex
+{
+public:
+    Regex(const string & pattern);
+    ~Regex();
+    bool matches(const string & s);
+
+private:
+    regex_t preg;
+    string showError(int err);
+};
+
+}