manpages: Remove telnetd references

Mostly remove from the SEE ALSO section, adding a mention of the port
where not removed. Elsewhere, remove as appropriate and change from .Xr
to .Nm where a mention of telnetd continues to make sense (or removing
it would require significant reworking of the surrounding text).

Reviewed by: imp, delphij, emaste
Differential Revision: https://reviews.freebsd.org/D36785