Fixed the error message in making a socket.
Fixed nexec to use a special file descriptor for writing.

Because start_slave closes given two file descriptors. At this time, if
two file descriptors are same (a socket is both readable and writable),
start_slave closes a closed file descriptor again. It causes "Bad file
descriptor" error.
Arranged CMakeLists.txt for nexecd.
Enhanced nexecd to execute /bin/echo.

This nexecd has some problems.

1. This version does not receive command line arguments from nexec. This
nexecd executes "/bin/echo foo" always.
2. This nexecd does not write any pid files.
3. Too slow to show "foo".