update README to reflect current porting approach

ok beck@ deraadt@
override native arc4random_buf on OS X

While the native OS X implementation is fork-safe, it does not seed
safely, as of the latest released OS X libc sources, version 997.90.3.
It only uses weak sources of entropy if accessing /dev/urandom fails.

ok beck@ deraadt@
override native arc4random_buf on FreeBSD

The FreeBSD-native arc4random_buf implementation falls back to weak
sources of entropy if the sysctl fails. Remove these dangerous fallbacks
by overriding locally.

Unfortunately, pthread_atfork() is broken on FreeBSD (at least 9 and 10)
if a program does not link to -lthr. Callbacks registered with
pthread_atfork() simply fail silently. So, it is not always possible to
detect a PID wraparound. I wish we could do better.

This improves arc4random_buf's safety compared to the native FreeBSD
implementation. Tested on FreeBSD 9 and 10.

ok beck@ deraadt@