nixos/nixos-option: Rewrite in a more suitable language

Also add --all, which shows the value of all options.  Diffing the --all
output on either side of contemplated changes is a lovely way to better
understand what's going on inside nixos.

nixos/modules/installer/tools/nixos-option/libnix-copy-paste.cc

@@ -0,0 +1,81 @@
+// These are useful methods inside the nix library that ought to be exported.
+// Since they are not, copy/paste them here.
+// TODO: Delete these and use the ones in the library as they become available.
+
+#include <nix/config.h> // for nix/globals.hh's reference to SYSTEM
+
+#include "libnix-copy-paste.hh"
+#include <boost/format/alt_sstream.hpp>           // for basic_altstringbuf...
+#include <boost/format/alt_sstream_impl.hpp>      // for basic_altstringbuf...
+#include <boost/format/format_class.hpp>          // for basic_format
+#include <boost/format/format_fwd.hpp>            // for format
+#include <boost/format/format_implementation.hpp> // for basic_format::basi...
+#include <boost/optional/optional.hpp>            // for get_pointer
+#include <iostream>                               // for operator<<, basic_...
+#include <nix/types.hh>                           // for Strings, Error
+#include <string>                                 // for string, basic_string
+
+using boost::format;
+using nix::Error;
+using nix::Strings;
+using std::string;
+
+// From nix/src/libexpr/attr-path.cc
+Strings parseAttrPath(const string &s) {
+  Strings res;
+  string cur;
+  string::const_iterator i = s.begin();
+  while (i != s.end()) {
+    if (*i == '.') {
+      res.push_back(cur);
+      cur.clear();
+    } else if (*i == '"') {
+      ++i;
+      while (1) {
+        if (i == s.end())
+          throw Error(format("missing closing quote in selection path '%1%'") %
+                      s);
+        if (*i == '"')
+          break;
+        cur.push_back(*i++);
+      }
+    } else
+      cur.push_back(*i);
+    ++i;
+  }
+  if (!cur.empty())
+    res.push_back(cur);
+  return res;
+}
+
+// From nix/src/nix/repl.cc
+bool isVarName(const string &s) {
+  if (s.size() == 0)
+    return false;
+  char c = s[0];
+  if ((c >= '0' && c <= '9') || c == '-' || c == '\'')
+    return false;
+  for (auto &i : s)
+    if (!((i >= 'a' && i <= 'z') || (i >= 'A' && i <= 'Z') ||
+          (i >= '0' && i <= '9') || i == '_' || i == '-' || i == '\''))
+      return false;
+  return true;
+}
+
+// From nix/src/nix/repl.cc
+std::ostream &printStringValue(std::ostream &str, const char *string) {
+  str << "\"";
+  for (const char *i = string; *i; i++)
+    if (*i == '\"' || *i == '\\')
+      str << "\\" << *i;
+    else if (*i == '\n')
+      str << "\\n";
+    else if (*i == '\r')
+      str << "\\r";
+    else if (*i == '\t')
+      str << "\\t";
+    else
+      str << *i;
+  str << "\"";
+  return str;
+}