btrfs-progs: utils: Introduce new pseudo random API

btrfs-progs: utils: Introduce new pseudo random API

David has reported some quite chaos usage of pseudo random numbers.
Like using static srand seed, or even calling rand() without setting
seed correctly.

The new pseudo random API will initialize the random seed on its first
calling and use uniformly distributed pseudo random number generator as
backend.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[ renamed variables and functions, added prefixes ]
Signed-off-by: David Sterba <dsterba@suse.com>