Minor: dmtcp_get_restart_env, add plugin-tutorial


git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2320 ba6dc0ae-232f-0410-883f-9ebac64fc565
modify-environ: Allow $ env var expansion


git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2321 ba6dc0ae-232f-0410-883f-9ebac64fc565