Vfork support broken.

[karya0]

Vfork support requires some refactoring of plugin layers wrt. PRESUSPEND barrier. The make-check-vfork* tests are disabled for now.

If someone requires vfork support, please upvote this issue.

[Larofeticus]

I'd like to have the vfork support in dmtcp.

I'm working in the area of using dmtcp with workflow management tools to preserve and resume their active state and child tasks. A significant number of tools I would like to support use vfork to launch their tasks (Snakemake, regular GNU Makefile, anything that uses the python subprocess package) are currently unusable with dmtcp.

The outcome of trying is the workflow tool silently hangs when reaching the vfork call; there is no stderr or exit code.

[karya0]

@Larofeticus: I just created a simple PR (#1023) to simulate vfork using fork(). Can you give it a try and see if your usecase is satisfied? If it works, then we can merge it in.