Add x86 to meterpreter's binary suffix

This makes x86 more consistent with x64.

Also replaces a bunch of instances of:
  File.join(Msf::Config.install_root, 'data', ...)
with the simpler
  File.join(Msf::Config.data_directory, ...)

[See rapid7/meterpreter#19]
Land #2378, meterpreter DLL suffix

Tested in the following configurations:

* WinXP SP0 x86 - reverse_http, reverse_tcp, ms08-067, ms03-026
* Win7 x64 fully patched - reverse_https, reverse_tcp, x64/reverse_tcp

Tested with all public extensions. Behaviour matches that of the currently released MSF.

x64 binaries no longer show up in auto-complete for x86.