move all internal scripts to /usr/libexec - these aren't meant to be used directly

[Antoine Martin]

Only a few scripts should remain in /usr/bin:

• winswitch_applet / winswitch_server : essentials
• winswitch_away / winswitch_back : can be used by blueproximity, etc
• winswitch_stdio_socket / winswitch_stdio_tcp : can be used by users for diagnosys (maybe)

Almost everything else we can move to libexec.
This is cleaner and better for packaging. Means we don't need so many manual pages.