Ringbuffer rework.

Ringbuffer rework.

Almost completely rewritten in order to:

- be able to use a unlimited number of ringbuffer files

  0 specified with -b argument or in the GUI, means that the number of file
  is unlimited.

  else the maximum number of ring buffer files is arbitrarily set to 1024.

- close the current file and open (truncating it) the next file at switch

- set the final file name once open (or reopen)

- avoid the deletion of files that could not be truncated (can't arise now)
  and do not erase empty files

The idea behind that is to remove the limitation of the maximum # of
ringbuffer files being less than the maximum # of open fd per process
and to be able to reduce the amount of virtual memory usage (having only
one file open at most) or the amount of file system usage (by truncating
the files at switch and not the capture stop, and by closing them which
makes possible their move or deletion after a switch).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@7912 f5534014-38df-0310-8fa8-9805f1628bb7