linux-user uses the rand function for generating the value of the AT_RANDOM elf aux vector entry, and explicitly seeds the random number generator with the current time. This makes it impossible to reproduce runs that use the AT_RANDOM bytes.
This patch adds a command line option and a matching environment variable for setting the random seed, so that the AT_RANDOM values can be predictable when the user chooses. The default is still to seed the random number generator with the current time. This is an updated version of the patch, addressing a review comment from Eric Blake on version 3.
