net-mgmt/check_mk_agent: update to 2.3.0p12

Add dependencies on sysutils/ipmitool, devel/libstatgrab, ftp/wget,
and USES=python since those are listed as required in the docs.
[https://docs.checkmk.com/latest/en/agent_freebsd.html]
Switch to PREFIX (from LOCALBASE).
Change MK_VARDIR from /var/lib/check_mk_agent to /var/db/check_mk_agent.
Update the pkg-message to remove reference to devel/libstatgrab and
devel/libstatgrab since those are included now, and changed the docs URL
to be current.

Changelog: https://checkmk.com/werks

PR: 280953
Approved by: submitter is maintainer