Add osx agent, OSX-compatible mk-job

[catchdave]

Better OSX agent:

• I adapted the OSX version of the agent originally found at: ThomasKaiser/Check_MK. This seems to be more modern that the version in this repo, I believe.
• I added OSX support for lnx_if since the original netctr is no longer supported in check_mk.
• I fixed a bug that breaks smartctl stats for NVME drives in OSX
• Added a OSX version for mk-job (<<<job>>>). Different than original since head is non-GNU

Modified mk-job for OSX

• OSX is BSD-based and therefore time does not work as written. It is common for OSX systems to use brew to install GNU-tools, so mk-job will now work as long as gnu-time is installed.

Installer for OSX agent
I could not find instructions for installing the OSX agent anywhere. So I put together a simple script that I used to install on my mac machines. I have done ZERO testing outside of my 2 machines.

[catchdave]

This is new code added by me: Add OSX version of mk-job

[catchdave]

This is new code: replace netctr with osx version of lnx_if

[jonaskluger]

Hello catchdave,

thank you for this pull request. We will have a closer look at your patch and discuss internally on how and if we can implement it into the official code base.

Internal Ref: CMK-4103

[Notausstieg0309]

I have tested your proposed MacOS Agent on my MacMini's with macOS 10.15.4.

Regarding the lnx_if: Your implementation does not work for my machines as they have no homebrew software installed. So I have used the implementation from #180 together with your agent, as it supports plugins which is essential to me.

[Notausstieg0309]

I would suggest some slight changes to get a bit closer to the linux agent implementation/behavior.

[Notausstieg0309]

Please honor the environment before using the default paths as the official linux agent does as well:

Suggested change

	export MK_LIBDIR='/usr/local/lib/check_mk_agent'
	export MK_CONFDIR='/etc/check_mk'
	export MK_VARDIR='/var/lib/check_mk_agent'
	export MK_LIBDIR=${MK_LIBDIR:-/usr/local/lib/check_mk_agent}
	export MK_CONFDIR=${MK_CONFDIR:-/etc/check_mk}
	export MK_VARDIR=${MK_VARDIR:-/var/lib/check_mk_agent}

These paths can be configured via the launch daemon definition to a custom path without modifying the agent.

[Notausstieg0309]

According to Linux Agent, this line should be changed by inserting and using the inpath() function as in the Linux agent:

# Function to replace "if type [somecmd]" idiom
# 'command -v' tends to be more robust vs 'which' and 'type' based tests
inpath() {
    command -v "${1:?No command to test}" >/dev/null 2>&1
}

[mo-ki]

Hi catchdave,

I am sorry to close this PR, in particular because it took us so long.
I am not closing it, because it is awful, but because it is too large to asses it thoroughly. We currently don't have a bigger OSX environment to test it (and, judging from your comment, neither do you). Since I don't expect anyone to be confident enough to merge this in the next few years, I think closing it is the honest thing to do.
At the very least the change in mk-job is worth a dedicated PR, I think.

Cheers,
Moritz