Fix "tinc start" on Windows when the path contains spaces.

When invoking "tinc start" with spaces in the path, the following
happens:

    > "c:\Program Files (x86)\tinc\tinc.exe" start
    c:\Program: unrecognized argument 'Files'
    Try `c:\Program --help' for more information.

This is caused by inconsistent handling of command line strings between
execvp() and the spawned process' CRT, as documented on MSDN:
http://msdn.microsoft.com/library/431x4c1w.aspx