Update msfupdate to work with debian packages

If apt was used to install framework, use apt-get to update.
Merges msfupdate for debian.

Tested against:

  * Windows 7 (Community Edition installation)
  * Ubuntu 12 (CE install)
  * svn (vintage, from r10850, 2010-10-30)
  * git (a few days old)
  * BT5R3 (updated already via apt-get)

In all cases, the msfupdate performed as expected.