allow shutdown-hook to run functions

[roccoblues]

This adds support for function execution (exec sync-mailbox) to the shutdown-hook.

Notes:

• only Index and "Global" functions are supported

• the functions are only exectured after the whole hook is parsed. So in this example:

  shutdown-hook "echo 'before'; exec sync-mailbox; echo 'after'"
  

  the order of execution will be:

  1. before
  2. after
  3. sync-mailbox

• run_events() is a terrible name and the method is probably in the wrong place

Related to: #4184

[gahr]

the functions are only exectured after the whole hook is parsed. So in this example:
shutdown-hook "echo 'before'; exec sync-mailbox; echo 'after'"
the order of execution will be:

before
after
sync-mailbox

Not a fan of this "feature" :) Can we do better?

[roccoblues]

the functions are only exectured after the whole hook is parsed. So in this example:
shutdown-hook "echo 'before'; exec sync-mailbox; echo 'after'"
the order of execution will be:
before
after
sync-mailbox

Not a fan of this "feature" :) Can we do better?

yeah, same here. But fixing this will require bigger changes. Because the code that parses the hooks and exec statements is pretty low level. I'll have another look. If you have any ideas - shoot. ;-)