Running a local exploit like a post is not currently supported,
we should at least raise a warning or something, and not just
let it backtrace and confuse the user.
Also replaces dead code in lib/msf/core/exploit/local.rb with what was
actually being used for the Exploit::Local class that lived in
lib/msf/core/exploit.rb.
This can currently build an executable that creates a socket, opens a
temporary file, truncates that file with ftruncate(2) and calls
sendfile. Still needs to mmap NULL and figure out ring0 shellcode.
Baby steps.