Add deprecation notices for all rcmd tools

[allanjude]

Summary

These tools will be removed from head. This warning will be MFC'd to
stable/10 and stable/11 in time for their next releases.

[emaste]

Do we want to be explicit about being removed from future versions of the FreeBSD base system?

[emaste]

Also should we mention installing the package? Maybe the net/bsdrcmds port or package? I'm not sure what would be most clear there.

[allanjude]

In D11471#237195, @emaste wrote:

Do we want to be explicit about being removed from future versions of the FreeBSD base system?

I thought "base system" was redundant, but I see how it might be better to include it.

[emaste]

Indeed; ports/packages are very much part of FreeBSD.

[allanjude]

Update with feedback from emaste

[emaste]

This version seems fine to me.

[jlh]

Thanks Allan!

[peter]

rwho / ruptime are *not* part of the rcmd suite.

rmcd(3) suite is: rsh, rlogin, rsh

rwho / ruptime are *not* in any way related to this.

[jlh]

FYI this was submitted as https://svnweb.freebsd.org/base?view=revision&revision=322277