Fix some bugs in preparation for factorizing

* Stop removing \x0a characters with String#scan, which of course breaks
  the shellcode
* Fork so the original session continues to work
Use a proper main signature with arguments

Allows us to `unlink(argv[0])`
Add some entropy

* Random filename
* Stop shipping debug strings to the exploit executable

Also makes the writable path configurable, so we don't always have to
use /tmp in case it is mounted noexec, etc.
Factorize sock_sendpage
Land #2505, missing source fix for sock_sendpage