cmds: handle interrupted system calls in platform.node()

EINTR happens.

This is super rare, but I ran into it on Mac OS X 10.8.  The sign-off
action (ctrl+i) failed and the traceback dialog showed that
platform.node() raised because os.uname() raises.  Add an interruptable
wrapper for platform.node() and use it in the SignOff command.

Signed-off-by: David Aguilar <davvid@gmail.com>