Refactor {client,server}_random to call an intermediate function

I'll be using this to make an option for randomizing the time.
Control sending time with SSL_SEND_{CLIENT,SERVER}RANDOM_MODE

(I'd rather use an option, but it appears that the options field is
full.)

Now, we send the time in the gmt_unix_time field if the appropriate
one of these mode options is set, but randomize the field if the flag
is not set.