This is a draft PR -- still a work in progress. But we can support full forked ckpt (including shared memory), with only a little extra code.

Support shared memory for forked ckpt

• mtcp_writememoryareas(int fd, int type):
  type = AREA_SHARED, AREA_NONSHARED, AREA_DONE, AREA_ALL
  Then write AREA_SHARED, then fork, and then AREA_NONSHARED.
  This assumes AREA_SHARED is smaller, and will not delay the parent,
  when writing this small amount of memory to the checkpoint image file.

ALSO: *.dmtcp.temp to *.dmtp only when done, and unlink previous ckpt file when we begin a new ckpt.

• This is to make sure that a user doesn't accidentally see the previous checkpoint image and think that the new one is done.
  But I should revise this. Instead of unlinking the previous ckpt file, I should rename it to *.dmtcp.old, and then unlink it only when the new ckpt image file has been created.

TODO:

• Verify that we are not doing any new mmap (e.g., JTRACE) in multiple calls to mtcp_writememoryareas()
• Improve shared-memory1 and shared-memory2 to verify that the ckpt image doesn't include new writes by the parent. Then manually test that, after configuring for --enable-forked-checkpointing.
• Maybe also have coordinator rename restart_script.sh to restart_script.sh.old and then unlink it when the ckpts are done?
  But this code would be ugly. So, maybe just let the user beware, and they can look for *.dmtcp.old instead.