Fix for rev 2750

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2753 ba6dc0ae-232f-0410-883f-9ebac64fc565
Remove dmtcp_set_tmpdir() from dmtcp.h

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2754 ba6dc0ae-232f-0410-883f-9ebac64fc565
Use protected-fd for lifeboat instead of env-var.

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2755 ba6dc0ae-232f-0410-883f-9ebac64fc565