When a fifo was closed, the fifo event handler would close the fd and
unregister it from the event handler, however the hook on BufClose
did that as well without checking if the fd was still refering to the
fifo.
Now we use a Buffer flag Fifo to tag the buffer as still linked to
a fifo so that the BufClose hook do not close and unregister a second
time
jump forward is bound to ctrl-i
jump backward is bound to ctrl-o
switch buffers or jumping somewhere in the buffer push the current
position to the jump list.
when a buffer is deleted, all entries referencing it in jump lists
are erased.
