Show selected module action

**Feature description:**

```show options``` now displays the selected action when an auxiliary or post module is being used. Previously, it would show only the selected target if an exploit module was being used.

This PR also fixes tab completion for post module actions.

**Verification steps:**

- [x] ```use``` an auxiliary module with actions
- [x] ```show options``` and see that the selected action is displayed
- [x] ```use``` an auxiliary module without actions
- [x] ```show options``` and see that no selected action is displayed
- [x] ```use``` a post module with actions
- [x] ```set``` an action using tab completion and see that it now completes
- [x] ```show options``` and see that the selected action is displayed
- [x] ```use``` a post module without actions
- [x] ```show options``` and see that no selected action is displayed
- [x] ```use``` an exploit module
- [x] ```show options``` and see that the selected target is still displayed

**Sample run:**

```
msf > use auxiliary/admin/chromecast/chromecast_youtube
msf auxiliary(chromecast_youtube) > show options

Module options (auxiliary/admin/chromecast/chromecast_youtube):

   Name     Current Setting  Required  Description
   ----     ---------------  --------  -----------
   Proxies                   no        Use a proxy chain
   RHOST                     yes       The target address
   RPORT    8008             yes       The target port
   VHOST                     no        HTTP server virtual host
   VID      kxopViU98Xo      yes       Video ID


Auxiliary action:

   Name  Description
   ----  -----------
   Play  Play video


msf auxiliary(chromecast_youtube) > use post/linux/gather/enum_xchat 
msf post(enum_xchat) > show options 

Module options (post/linux/gather/enum_xchat):

   Name     Current Setting  Required  Description
   ----     ---------------  --------  -----------
   SESSION                   yes       The session to run this module on.


Post action:

   Name  Description
   ----  -----------
   ALL   Collect both the plists and chat logs


msf post(enum_xchat) >
```