crypt: Add workaround for non-fork-safe PRGs.

crypt: Add workaround for non-fork-safe PRGs.

Some PRNGs implementations suffer from the problem that after a fork()
the PRNG state of parent and child process differ only by the child pid
which is mixed into the state. Certain versions of openssl are known to
contain this flaw.

On such implementations two command handlers will generate the same
challenge and session keys if their pid is identical. This may happen
due to pid wrapping.

This patch works around this shortcoming by reading some pseudo random
bytes in the parent process after each fork().