Expose configuration functions for configuring entropy sources
Require that /dev/urandom replacement be a device file.
Allow urandom to be specified by fd.
Unit tests for urandom-by-fd
Add a state argument to entropy source functions
Check that urandom inode does not change on us.

Since we're allowing the user to pass /dev/urandom by fd, we need to
be sure that the fd is not replaced (by close, dup2, or whatever).
Merge branch 'topic/check_rng_device'
Clean the TODO