SECURITY: disable dumpable flag on Linux.

Otherwise client can modify /proc/self/mem and rewrite the server's
memory.

/proc/self/mem may be aliased so name-based checks are hopeless.
This mitigation is the same as used by OpenSSH's SFTP server.

Additional information:
  http://seclists.org/fulldisclosure/2014/Oct/35
Copyright date.
Release 0.2.1