Use wslpath to find the backend program

If the wslpath command is in the PATH, then wslbridge relies on it to find
the backend. If it's not in the PATH, then it falls back on its own
X:\foo\bar ==> /mnt/x/foo/bar path conversion.

I tested using the /etc/wsl.conf file here:
https://blogs.msdn.microsoft.com/commandline/2018/02/07/automatically-configuring-wsl/

If the /windir directory didn't exist, then no /windir/c mount was created,
and wslbridge failed with this error message:

$ ./out/wslbridge
wslbridge error: failed to start backend process
note: backend error output: wslpath: C:\rprichard\proj\wslbridge\out\wslbridge-backend: No such file or directory

If the directory does exist, then wslbridge finds the /windir/c/...
backend path.

Fixes #22

LICENSE.txt

@@ -1,6 +1,7 @@
 The MIT License (MIT)
 
 Copyright (c) 2016 Ryan Prichard
+Copyright (c) 2018 Google LLC
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to

frontend/wslbridge.cc

@@ -1092,7 +1092,12 @@ int main(int argc, char *argv[]) {
 
     // Prepare the backend command line.
     std::wstring bashCmdLine;
+    bashCmdLine.append(L"$(if [ \"$(command -v wslpath)\" ]; then wslpath");
+    appendBashArg(bashCmdLine, backendPathWin);
+    bashCmdLine.append(L" || echo false; else echo");
     appendBashArg(bashCmdLine, backendPathWsl);
+    bashCmdLine.append(L"; fi)");
+
     if (debugFork) {
         appendBashArg(bashCmdLine, L"--debug-fork");
     }