OK, ready for submission.

This patch adds two new functionalities for linux native debugging:
   * Multiple fork debugging, and
   * Checkpoint / Restart.

The full doc is included in the patch, but briefly, the user interface
looks like this:

     set/show detach-on-fork (on/off, default on)
	If off, gdb will not detach from either parent or child,
	but will debug both, holding all but one fork suspended.
     info forks
     fork <n> (analogous to thread <n>)
     process <pid> (select by pid instead of by fork id)
     detach-fork <n>
     delete-fork <n>

     checkpoint (no argument)
     info checkpoints
     restart <n>
     delete-checkpoint <n>

There's a test for multi-fork which debugs 16 "native" forks
(ie. forks actually created by the inferior), and a test for
checkpoints which at one point creates over 600 checkpoints
at once.  It also tests multiple forks that are simultaneously
reading and writing to/from the same files.