pgraph: Add fast pausing and exiting of graphs

This causes a graph to actually stop processing part way through, even
if there are poke's that want to continue on. This is so that the user
experience of pressing ^C actually causes a shutdown without finishing
the graph execution. It might be preferred to have this be a user
defined setting at some point in the future, such as if the user presses
^C twice. As well, we might want to implement an interrupt API so that
individual resource execution can be asked to bail out early if
requested. This could happen on a third ^C press.

test/shell/graph-exit1.yaml

@@ -0,0 +1,71 @@
+---
+graph: parallel
+resources:
+  exec:
+  - name: exec1
+    cmd: sleep 15s
+    shell: ''
+    timeout: 0
+    watchcmd: ''
+    watchshell: ''
+    ifcmd: ''
+    ifshell: ''
+    state: present
+  - name: exec2
+    cmd: sleep 15s
+    shell: ''
+    timeout: 0
+    watchcmd: ''
+    watchshell: ''
+    ifcmd: ''
+    ifshell: ''
+    state: present
+  - name: exec3
+    cmd: sleep 15s
+    shell: ''
+    timeout: 0
+    watchcmd: ''
+    watchshell: ''
+    ifcmd: ''
+    ifshell: ''
+    state: present
+  - name: exec4
+    cmd: sleep 15s
+    shell: ''
+    timeout: 0
+    watchcmd: ''
+    watchshell: ''
+    ifcmd: ''
+    ifshell: ''
+    state: present
+  - name: exec0
+    cmd: sleep 15s
+    shell: ''
+    timeout: 0
+    watchcmd: ''
+    watchshell: ''
+    ifcmd: ''
+    ifshell: ''
+    state: present
+edges:
+- name: e1
+  from:
+    kind: exec
+    name: exec1
+  to:
+    kind: exec
+    name: exec2
+- name: e2
+  from:
+    kind: exec
+    name: exec2
+  to:
+    kind: exec
+    name: exec3
+- name: e3
+  from:
+    kind: exec
+    name: exec3
+  to:
+    kind: exec
+    name: exec4