init: clean up random seed options

init: clean up random seed options

- make allrandrepeat a synonym of randrepeat. allrandrepeat is
  superfluous because the seeds set by randrepeat already encompass
  random number generators beyond the one used for random offsets.
- allow randseed to override [all]randrepeat: this is what the
  documentation implies but was not previously the case

This is a breaking change for users relying on the values of fio's
default random seeds.

Link: https://github.com/axboe/fio/pull/1546
Fixes: https://github.com/axboe/fio/issues/1502
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>