Add sysctls for arc shrinking and growing values

The default value for arc_no_grow_shift may not be optimal when using
several GiB ARC. Expose it via sysctl allows users to tune it easily.

Also expose arc_grow_retry via sysctl for the same reason. The default value of
60s might, in case of intensive load, be too long.

Submitted by: Nikita Kozlov <nikita.kozlov@blade-group.com>
Reviewed by: mav, manu, bapt
MFC after: 2 weeks
Sponsored by: blade
Differential Revision: https://reviews.freebsd.org/D12144