This commit is the main infrastructure needed to run exploits in a local
context, gluing the Exploit and Post module classes together.

Tested on Ubuntu with nmap 6.00 and nmap 5.00

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.

Something is still broken, my socket() is returning EAFNOSUPPORT whereas
what looks like the same syscall in wunderbar_emporium's exploit.c is
returning a socket. Similarly, my __mmap2() is returning EFAULT when
trying to map anything, not just NULL.

Unfortunately, adds a dep on bionic for runtime compilation.

Gets ring0, sets the (res)uid to 0 and jumps to the payload.  Still some
payload issues because linux stagers don't mprotect(2) the buffer they
read(2) into.  Single payloads work fine, though.

Also cleans up and improves local exploits' ability to compile C.

[SEERM rapid7#3038]