Fix vulnerability in process_sigaction()
Fix vulnerability in process_sigaction()

This fixes a bug in which the trusted process will forward an
unchecked syscall number, received from an untrusted thread, for
execution by a trusted thread.  Add a check for this syscall
number.

BUG=http://code.google.com/p/seccompsandbox/issues/detail?id=6
TEST=None: Although we could write a test for this check, in the
  longer term we should probably refactor the message format to
  remove the duplicated syscall number.

Review URL: http://codereview.chromium.org/3542001
cnbird1999/seccompsandbox,ericmckean/seccompsandbox,ericmckean/seccompsandbox,cnbird1999/seccompsandbox,ericmckean/seccompsandbox,cnbird1999/seccompsandbox