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]

## Verification
 - [ ] in 32-bit session, tab complete an extension, e.g. "load incog<tab>"; before: see `incognito` and `incognito.x64`; after see just `incognito`
 - [ ] `load incognito`; verify that incognito commands still work
 - [ ] migrate to another process; verify that correct dll is used and session still functions afterward
 - [ ] `screenshot` in 32-bit session; verify a jpeg is downloaded
 - [ ] `screenshot` in 64-bit session; verify a jpeg is downloaded