[sestarnix] Implement the dmesg_restrict kernel option

By default Starnix restricts access to the syslog/kmsg to callers
with the relevant capability (SYS_ADMIN or SYSLOG).

If dmesg_restrict is set to zero then unprivileged callers may
access the syslog/kmsg.

Bug: 316630310
Change-Id: Iae7efa370f76619ee354638dcaf70e69f3020055
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1228386
Reviewed-by: Miguel Flores <miguelfrde@google.com>
Fuchsia-Auto-Submit: Wez <wez@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>