Allow setting a set of errnos rather than just a particular errno or a
random one from the entire set. Update the cookbook for random faults to
exclude any errnos passed via extra arguments.

My objective here is to be able to exclude a specific errno from
random injection. The Go runtime gets confused by EAGAINs, which cause
it to epoll_wait on the file descriptor. I'd like to exclude EAGAIN from
the set of injected errors for my use case.

The CockroachDB test suite currently makes use of charybdefs-nemesis to
induce artificial delays into the filesystem to simulate disk stalls.
Currently, the delay duration is fixed at 50ms. This precludes testing
of faults that last indefinitely.

Change the format of the `--delay` command to take a duration, in
microseconds, allowing for faults of a longer duration.