enable the asn1 test
register the atfork handler from arc4random

From kettenis@

People have suggested using pthread_atfork(3) before, but discarded
the idea because it involves linking with -lpthread, which has other
undesirable consequences.  However:

* Most systems actually have pthread_atfork(3) in libc.  I verified
  this on OS X and Solaris.  I believe this is the case on Linux
  systems that use musl as well.

* On Linux systems that use glibc, this isn't the case.  However,
  those systems have __register_atfork(3), which is fully documented
  in the "Linux Standard Base Core Specification".

ok kettenis@ deraadt@ beck@