Fix for dmtcp_launch --{ptrace,ib,rm,modify-env,alloc}

- These flags were not preserved when creating a remote ssh child.
- The fix is to detect these plugins and add the flags when launching
  ssh child.

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2451 ba6dc0ae-232f-0410-883f-9ebac64fc565