Rename signals update and eof

Rename the libsigc++ signals to signal_update and signal_eof to match
the naming used for signals in GParted.
    fgrep 'sigc::signal' include/*.h

Also explicitly use the emit() method rather than using the object
operator().  This again is to match the convention in GParted and make
it more obvious what is happening.
    fgrep '.emit(' include/*.h