OSX post module: password_prompt_spoof

RM #5940

This module pops up a password dialog to a logged in OSX user.

### Verification
- [ ] Gain yourself one shell session on an OSX box.
- [ ] Run the exploit:
     `use post/osx/gather/password_prompt_spoof`
     `set SESSION <id>`
     `run`
- [ ] On the OSX machine, verify that a password dialog appears. Enter a password end click OK.
- [ ] Verify that loot is saved containing the password.
- [ ] Repeat with a java-meterpreter session on an OSX box.