Remove remnants from RC2 and SEED - there are no longer any cipher suites
that use these algorithms (and SEED was removed from libcrypto some time
ago).

ok doug@
Add a tls_connect_fds() function that allows a secure connection to be
established using a pair of existing file descriptors.

Based on a diff/request from Jan Klemkow.

Rides previous libtls rename/library bump.

Discussed with tedu@.
Add hooks to 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 also 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.