staging: usbip: add "-P" / "--pid" option to save usbipd process id

Introduce option "-P" / "--pid" to request that usbipd save its PID to
a file while running.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>